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

[alexcrichton]

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 by 30% from 1s to
0.7s.

[rust-highfive]

r? @brson

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[TimNN]

Additionally #[inline] is an unnecessary annoation on almost all
generic functions because the function will already be monomorphized into other
codegen units

I'm not exactly sure, but is this still true? If I recall correctly, there were some changes (to the collector?) a while ago, which changed the behaviour here

[oyvindln]

I know that at least that a adding inline to try_from helped that get inlined.

[alexcrichton]

@TimNN ah yes I always forget that #[inline] affects monomorphization in codegen units as well. I've updated the PR to just downgrade #[inline(always)] to #[inline] in a number of cases then.

[alexcrichton]

r? @sfackler

[sfackler]

@bors r+

[bors]

📌 Commit 53d8b1d has been approved by sfackler

[bors]

⌛ Testing commit 53d8b1d with merge f8d485f...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: sfackler
Pushing f8d485f to master...