Auto merge of #53659 - nnethercote:rm-AccumulateVec, r=Mark-Simulacrum

Remove `AccumulateVec` and its uses.

It's basically just a less capable version of `SmallVec`.

FWIW, the only use of `ArrayVec` is now within `HybridIdxSet`.

r? @Mark-Simulacrum

diff --cc src/librustc/ty/context.rs
Simple merge

diff --cc src/librustc/ty/structural_impls.rs
Simple merge