Skip conversion of contexts into vectors

[crisptrutski]

It turns out there's a big opportunity to expose the scopes directly as Java objects rather than vectors.

I tried instead just interning the conversions, but that turned out quite complex and to still use quite a bit more memory. Since we don't actually use the contexts for anything externally, this seems fine. The API has added some methods to read their properties without interop.

This is a non-breaking change for Metabase.

Before all these context optimizations:

> (user/time+ (complex-benchmark))
Time per call: 77.74 ms   Alloc per call: 111,128,445b   Iterations: 26

Afterwards (incl previous PR)

> (user/time+ (complex-benchmark))
Time per call: 43.94 ms   Alloc per call: 29,220,062b   Iterations: 47

[crisptrutski]

A big part of the win is probably not building new sequences. I tried caching the cons -> sequence mappings as well and also didn't help much.