Auto merge of #107167 - the8472:rawvec-simpler-layout, r=thomcc

simplify layout calculations in rawvec

The use of `Layout::array` was introduced in #83706 which lead to a [perf regression](rust-lang/rust#83706 (comment)).

This PR basically reverts that change since rust currently only supports stride == size types, but to be on the safe side it leaves a const-assert there to make sure this gets caught if those assumptions ever change.

Author: bors