Use a `BitSet` in `LexicalResolver::iterate_until_fixed_point()`.
Use a `BitSet` in `LexicalResolver::iterate_until_fixed_point()`.

This wins 3% on `unicode_normalization`.

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust