Auto merge of rust-lang#145480 - cjgillot:sizeless-chunks, r=Mark-Simulacrum

Remove chunk size from each chunk in `ChunkedBitSet`.

Almost all chunks in a `ChunkedBitSet` have the same constant `CHUNK_SIZE`, except the last in a bitset which takes the remainder `domain_size % CHUNK_SIZE`.

r? `@ghost` for perf

Author: bors