[Experiment] Trivialize save points

[milseman]

Make SavePoint a trivial value rather than one containing strong references.

[milseman]

Long-term we want to move in this direction, but it will take some significant progress elsewhere before it makes sense. Currently yields lots of regressions.

=== Regressions ======================================================================
- EmailRFCNoMatches_All_Scalar            138ms	126ms	12.3ms		9.8%
- EmailRFC_All_Scalar                     55.8ms	47ms	8.78ms		18.7%
- EmailRFC_All                            72.1ms	63.7ms	8.45ms		13.3%
- EmailRFCNoMatches_All                   149ms	142ms	7.32ms		5.1%
- EmojiRegex_All_Scalar                   57ms	49.9ms	7.04ms		14.1%
- DiceRollsInText_All                     53.3ms	47ms	6.23ms		13.2%
- EmojiRegex_All                          79.2ms	73ms	6.17ms		8.5%
- DiceRollsInText_All_Scalar              50.4ms	44.3ms	6.16ms		13.9%
- IntersectionCCC_All_Scalar              23.7ms	21.9ms	1.8ms		8.2%
- IntersectionCCC_All                     23.7ms	21.9ms	1.8ms		8.2%
- SubtractionCCC_All                      23.2ms	21.4ms	1.79ms		8.4%
- SubtractionCCC_All_Scalar               23.1ms	21.5ms	1.69ms		7.9%
- EmailBuiltinCharacterClass_All_Scalar   13.8ms	12.6ms	1.27ms		10.1%
- EmailBuiltinCharacterClass_All          14ms	12.7ms	1.26ms		9.9%
- ReluctantQuant_Whole_Scalar             11.9ms	10.6ms	1.23ms		11.5%
- ReluctantQuant_Whole                    11.9ms	10.7ms	1.2ms		11.3%
- EmailLookahead_All_Scalar               23.8ms	22.7ms	1.14ms		5.0%
- symDiffCCC_All                          50.8ms	49.7ms	1.1ms		2.2%
- HangulSyllable_All_Scalar               7.47ms	6.38ms	1.09ms		17.1%
- EmailLookaheadNoMatches_All             42.3ms	41.3ms	1.02ms		2.5%
- BasicBuiltinCharacterClass_All          9.38ms	8.41ms	969µs		11.5%
- NotFound_All_Scalar                     7.45ms	6.51ms	942µs		14.5%
- EmailLookahead_All                      41.2ms	40.3ms	936µs		2.3%
- symDiffCCC_All_Scalar                   50.7ms	49.8ms	918µs		1.8%
- BasicBuiltinCharacterClass_All_Scalar   8.65ms	7.78ms	863µs		11.1%
- BasicCCC_All                            11.5ms	10.7ms	845µs		7.9%
- HangulSyllable_All                      7.97ms	7.12ms	843µs		11.8%
- ReluctantQuantWithTerminal_Whole        7.5ms	6.66ms	837µs		12.6%
- LiteralSearchNotFound_All_Scalar        6.78ms	5.96ms	820µs		13.8%
- LiteralSearch_All_Scalar                7.01ms	6.2ms	816µs		13.2%
- NotFound_All                            8.18ms	7.37ms	811µs		11.0%
- LiteralSearchNotFound_All               7.54ms	6.76ms	783µs		11.6%
- LiteralSearch_All                       7.75ms	6.98ms	771µs		11.1%
- EmailLookaheadNoMatches_All_Scalar      28.7ms	28ms	739µs		2.6%
- ReluctantQuantWithTerminal_Whole_Scalar 7.46ms	6.75ms	712µs		10.6%
- CaseInsensitiveCCC_All                  12.5ms	11.8ms	632µs		5.3%
- DiceNotation_Scalar                     5.29ms	4.66ms	628µs		13.5%
- BasicCCC_All_Scalar                     11.3ms	10.7ms	609µs		5.7%
- CaseInsensitiveCCC_All_Scalar           12.4ms	11.8ms	607µs		5.1%
- BasicRangeCCC_All                       11.7ms	11.1ms	578µs		5.2%
- BasicRangeCCC_All_Scalar                11.7ms	11.1ms	574µs		5.2%
- DiceNotation                            5.58ms	5.03ms	547µs		10.9%
- HangulSyllable_First                    3.85ms	3.41ms	440µs		12.9%
- HangulSyllable_First_Scalar             3.49ms	3.06ms	433µs		14.2%
- Words_All_Scalar                        13.5ms	13.1ms	371µs		2.8%
- EmailLookaheadList                      10ms	9.66ms	348µs		3.6%
- Numbers_All                             7.52ms	7.17ms	344µs		4.8%
- InvertedCCC_All                         20.9ms	20.6ms	332µs		1.6%
- MACAddress_Scalar                       2.82ms	2.5ms	320µs		12.8%
- EmailLookaheadList_Scalar               5.54ms	5.22ms	314µs		6.0%
- MACAddress                              3.19ms	2.89ms	301µs		10.4%
- Words_All                               13.9ms	13.7ms	257µs		1.9%
- Numbers_All_Scalar                      6.97ms	6.73ms	245µs		3.6%
- IPv6Address_Scalar                      3.02ms	2.8ms	221µs		7.9%
- IPv4Address_Scalar                      2.45ms	2.25ms	206µs		9.2%
- Css_All_Scalar                          3.39ms	3.23ms	163µs		5.0%
- IPv6Address                             4.06ms	3.9ms	160µs		4.1%
- IPv4Address                             2.67ms	2.52ms	153µs		6.1%
- Css_All                                 3.9ms	3.78ms	126µs		3.3%
- Lines_All_Scalar                        1.83ms	1.78ms	50.1µs		2.8%
- Lines_All                               1.88ms	1.84ms	40.7µs		2.2%
=== Improvements =====================================================================
- CompilerMessages_All_Scalar             79.1ms	85.4ms	-6.26ms		-7.3%
- CompilerMessages_All                    96.1ms	102ms	-5.98ms		-5.9%
- EagarQuantWithTerminal_Whole            793µs	1.42ms	-626µs		-44.1%
- EagarQuantWithTerminal_Whole_Scalar     789µs	1.4ms	-607µs		-43.5%
- AnchoredNotFound_Whole                  8.8ms	8.9ms	-107µs		-1.2%
- GraphemeBreakNoCap_All_Scalar           3.62ms	3.72ms	-98.8µs		-2.7%
- AnchoredNotFound_Whole_Scalar           5.58ms	5.67ms	-91µs		-1.6%
- GraphemeBreakNoCap_All                  4.24ms	4.31ms	-71.8µs		-1.7%

Things to address:

• Quantified save points need an overhaul
  • We create save points that we never push back
  • Instead we should just locally update a range
  • This is what I'm looking at next
• Bookkeeping logic to share saved states across saved points
  • Don't push if there's been no mutating in state
  • On restore, don't pop if still being used by some save point
  • Consider per-capture indices, per-register-bank indices, etc.
• Tweaks
  • Optimizer can be especially finicky around ARC, especially as we migrate to structs and the inside of signalFailure()
    • This was the reason we have destructure rather than member access
  • Improve code layout so that a save point fits in a few words
  • Get off of mutable integer registers and/or COW them differently
• Further architectural/design work around unique values
  • Look into ~Copyable or ~Escaping types and non-escaping parameters
  • Shrink Processor or otherwise eliminate struct copies and retains