Remove allocation for builder cache hits

[CAD97]

Credit to @simonvandel over at the main rowan repository for pointing out how this is possible. Previously: #40

Now that the necessary APIs for this are riding the trains to stable, here's an actual decent implementation of alloc-free cache hits!

Also adds a bit more explicit typing on pointers, as that led to a bug in the construction of this patch.

[codecov]

Codecov Report

Merging #45 into master will decrease coverage by 0.68%.
The diff coverage is 81.08%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
- Coverage   82.88%   82.19%   -0.69%     
==========================================
  Files          10       10              
  Lines         707      747      +40     
==========================================
+ Hits          586      614      +28     
- Misses        121      133      +12     

Impacted Files	Coverage Δ
src/green/tree_builder.rs	61.22% <ø> (ø)
src/utils.rs	66.66% <47.36%> (-12.29%)	⬇️
src/green/builder.rs	86.40% <92.72%> (+1.59%)	⬆️

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[CAD97]

miri failure is an expected build failure, as miri is not available on the current nightly (of which we're using stabilizations from). https://rust-lang.github.io/rustup-components-history/

[CAD97]

bors: r+

This is nightly-only currently, but riding the trains to stable. However, this is complicated enough (and needed enough) that I'd rather not have to keep re-rebasing while making other improvements.

[bors]

Build succeeded:

• Clippy
• Miri
• Tests
• Rustfmt