Makes the behavoral change of #13263 without the underlying refactor.
Hopefully this clearly safe from a perf and GC perspective, and will
make it easier to benchmark #13263.
1. Fix confusing wording that might imply unnecessary double-hashing.
2. Add references to specifics of base-32 encoding.
3. Fix incorrect description that sha256 hash of `fingerprint` is
truncated. "Truncated" is actual wording used in Nix theses, but it has
unusual meaning, that is better conveyed by word "compressed", which is
used by the reference C++ implementation.
4. Clarify details of base16 encoding.
This is a big step documenting the store layer on its own, separately from the evaluator (and `builtins.derivation`).
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
