Remove bitcode::__private usage, switch to derive macros

[singulared]

Replace ~280 lines of manual `bitcode::__private` codec with derive macros for `CacheKey`/`KeyPart` encoding. Changes wire format — existing cache keys will miss (not corrupt).

Breaking: wire format change.

[AndreyErmilov]

Waiting for the merge of SoftbearStudios/bitcode#107