Auto merge of #126793 - saethlin:mono-rawvec, r=scottmcm

Apply "polymorphization at home" to RawVec

The idea here is to move all the logic in RawVec into functions with explicit size and alignment parameters. This should eliminate all the fussing about how tweaking RawVec code produces large swings in compile times.

This uncovered rust-lang/rust-clippy#12979, so I've modified the relevant test in a way that tries to preserve the spirit of the test without tripping the ICE.

Author: bors