std: Move running_on_valgrind to rt::util. #1457

[breaking-change]
std: Move simd to core::simd and reexport. #1457

[breaking-change]
std: Move unstable::finally to std::finally. #1457

[breaking-change]
core: Document simd mod
core: Derive Show on SIMD types
auto merge of #14379 : brson/rust/simd, r=alexcrichton

Followup to https://github.com/mozilla/rust/pull/14331 and https://github.com/mozilla/rust/pull/12524