Move from #[inline(always)] to #[inline]

[alexcrichton]

This commit blanket changes all #[inline(always)] annotations to #[inline].
Fear not though, this should not be a regression! To clarify, though, this
change is done out of correctness to ensure that we don't hit stray LLVM errors.

Most of the LLVM intrinsics and various LLVM functions we actually lower down to
only work correctly if they are invoked from a function with an appropriate
target feature set. For example if we were to out-of-the-blue invoke an AVX
intrinsic then we get a codegen error. This error comes about
because the surrounding function isn't enabling the AVX feature. Now in general
we don't have a lot of control over how this crate is consumed by downstream
crates. It'd be a pretty bad mistake if all mistakes showed up as scary
un-debuggable codegen errors in LLVM!

On the other side of this issue we as the invokers of these intrinsics are
"doing the right thing". All our functions in this crate are tagged
appropriately with target features to be codegen'd correctly. Indeed we have
plenty of tests asserting that we can codegen everything across multiple
platforms!

The error comes about here because of precisely the #[inline(always)]
attribute. Typically LLVM won't inline functions across target feature sets.
For example if you have a normal function which calls a function that enables
AVX2, then the target, no matter how small, won't be inlined into the caller.
This is done for correctness (register preserving and all that) but is also how
these codegen errors are prevented in practice.

Now we as stdsimd, however, are currently tagging all functions with "always
inline this, no matter what". That ends up, apparently, bypassing the logic of
"is this even possible to inline". In turn we start inlining things like AVX
intrinsics into functions that can't actually call AVX intrinsics, creating
codegen errors at compile time.

So with all that motivation, this commit switches to the normal inline hints for
these functions, just #[inline], instead of #[inline(always)]. Now for the
stdsimd crate it is absolutely critical that all functions are inlined to have
good performance. Using #[inline], however, shouldn't hamper that!

The compiler will recognize the #[inline] attribute and make sure that each of
these functions is candidate to being inlined into any and all downstream
codegen units. (aka if we were missing #[inline] then LLVM wouldn't even know
the definition to inline most of the time). After that, though, we're relying on
LLVM to naturally inline these functions as opposed to forcing it to do so.
Typically, however, these intrinsics are one-liners and are trivially
inlineable, so I'd imagine that LLVM will go ahead and inline everything all
over the place.

All in all this change is brought about by #253 which noticed various codegen
errors. I originally thought it was due to ABI issues but turned out to be
wrong! (although that was also a bug which has since been resolved). In any case
after this change I was able to get the example in #253 to execute in both
release and debug mode.

Closes #253