Rollup merge of #65480 - nnethercote:rm-iterate_until_fixed_size, r=nikomatsakis
Rollup merge of #65480 - nnethercote:rm-iterate_until_fixed_size, r=nikomatsakis

Speed up `LexicalResolve::expansion()`

A couple of improvements that speed up `unicode_normalization` by about 4%. The first commit was enabled by the improvements to `BitSet` iteration in #65425.

r? @nikomatsakis

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