Shrink AV by 8 bytes and AT by 16

[Centril]

Description of Changes

This primarily shrinks the size of types AlgebraicValue from 32 bytes to 24 and AlgebraicType from 32 to 16.
The size of some other types are also shrunk, notably, table, column, index, sequence, etc. definitions are shrunk by moving from String to Box<str>. Moreover, ProductValue is shrunk to 16 bytes, a type that is used frequently in hot places.

Benchmarks below relative to based master, run on i7-7700K, 64GB.

full-scan               time:   [80.111 ms 80.529 ms 80.979 ms]
                        change: [-14.103% -13.427% -12.763%] (p = 0.00 < 0.05)
                        Performance has improved.

Benchmarking full-join: Collecting 100 samples in estimated 6.
full-join               time:   [301.14 µs 302.01 µs 303.25 µs]
                        change: [-9.6948% -9.1117% -8.5214%] (p = 0.00 < 0.05)
                        Performance has improved.

Benchmarking incr-select: Collecting 100 samples in estimated 
incr-select             time:   [183.21 ns 185.12 ns 188.14 ns]
                        change: [-5.7687% -4.8383% -3.6110%] (p = 0.00 < 0.05)
                        Performance has improved.

API and ABI breaking changes

None

Expected complexity level and risk

2

Testing

No semantic changes, existing tests should suffice.

[kazimuth]

Very nice. I see why this took longer than expected, this is the sort of thing that gets everywhere.

My only real concern right now would be that this could result in $O(n^2)$ behavior for loops that repeatedly append to ArrayValues. I'm not aware of such loops off the top of my head, and I don't see any methods changed in this PR that expose such an interface for ArrayValue, so I don't think it's a problem.

[kazimuth]

This may inhibit SIMD optimizations for i/u128. Fortunately, that doesn't matter in the slightest -- packing AV and AT is way more important.

[cloutiertyler]

This is fine in principle, but let's be deferential to higher prio things about merging this.

[Centril]

Commitlog has merged, so going ahead and merging this.