feat(tazjin/rlox): Add a simple string interner

This is based on this matklad post:

https://matklad.github.io/2020/03/22/fast-simple-rust-interner.html

It's modified slightly to provide a safer interface and slightly more
readable implementation:

* interned string IDs are wrapped in a newtype that is not publicly
  constructible

* unsafe block is reduced to only the small scope in which it is
  needed

* lookup lifetime is pinned explicitly to make the intent clearer when
  reading this code

Change-Id: Ia3dae988f33f8e5e7d8dc0c1a9216914a945b036
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2578
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

users/tazjin/rlox/src/bytecode/mod.rs

@@ -5,6 +5,7 @@
 mod chunk;
 mod compiler;
 mod errors;
+mod interner;
 mod opcode;
 mod value;
 mod vm;