Auto merge of #43367 - alexcrichton:remove-inline-always, r=sfackler

std: Cut down #[inline] annotations where not necessary

This PR cuts down on a large number of `#[inline(always)]` and `#[inline]`
annotations in libcore for various core functions. The `#[inline(always)]`
annotation is almost never needed and is detrimental to debug build times as it
forces LLVM to perform inlining when it otherwise wouldn't need to in debug
builds. Additionally `#[inline]` is an unnecessary annoation on almost all
generic functions because the function will already be monomorphized into other
codegen units and otherwise rarely needs the extra "help" from us to tell LLVM
to inline something.

Overall this PR cut the compile time of a [microbenchmark][1] by 30% from 1s to
0.7s.

[1]: https://gist.github.com/alexcrichton/a7d70319a45aa60cf36a6a7bf540dd3a

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