compact: add Make methods for compact ranges

[pav-kv]

The New* methods unnecessarily allocate. If the user needs a pointer, they can convert, but most users probably only need it by value.

[pav-kv]

For reviewers: I don't feel strongly about this PR, but thought I'd throw it in for consideration. Been using this package for a side-project, and figured why not remove an allocation.

The current state is safer / avoids some footguns by design. So it's conceivable that it should stay this way.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.18%. Comparing base (4ebea17) to head (007cda6).
Report is 74 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #174       +/-   ##
===========================================
- Coverage   89.33%   58.18%   -31.16%     
===========================================
  Files           7        8        +1     
  Lines         497      782      +285     
===========================================
+ Hits          444      455       +11     
- Misses         48      322      +274     
  Partials        5        5               

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[mhutchinson]

Hey Pavel I intend to review this by the end of the week. I've just returned from vacation and need to get up to speed before deciding whether this subtle variation is worth the extra API method or not. Thanks for sending it!

[pav-kv]

@mhutchinson nvm, I think Go optimizes the allocation out if we say cr := *factory.NewRange(...):

diff --git a/compact/range_test.go b/compact/range_test.go
index 53bf58a..998c0cc 100644
--- a/compact/range_test.go
+++ b/compact/range_test.go
@@ -578,10 +578,10 @@ func TestDecomposePow2(t *testing.T) {
 
 func BenchmarkNewRange(b *testing.B) {
        b.ReportAllocs()
-       var cr *compact.Range
+       var cr compact.Range
        for range b.N {
-               // BenchmarkNewRange-10  69894982  17.13 ns/op  48 B/op  1 allocs/op
-               cr = factory.NewEmptyRange(10)
+               // BenchmarkNewRange-10  1000000000  0.3165 ns/op  0 B/op  0 allocs/op
+               cr = *factory.NewEmptyRange(10)
        }
        _ = cr
 }

[pav-kv]

I'll remove the API changes, and reduce this PR just to a bit of commenting in Range around the pointer/value stuff, since it still applies. Some time this/next week :)