test+fix: Add asm testing and fix core_hint_black_box
#147
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Per this zulip discussion and rust-lang/rust#140341 and in contrast to the discussion on #107,
read_volatile(and all other equivalent functions in the standard library) should provide strict fewer optimization barriers thanblack_box.In service of testing this, I've added a script which makes sure the assembly instructions of a simple case are approximately the expected length across most common architectures.
Also
read_volatileis never going to beconst, and it would be nice toconstify this library (which will hopefully be my next PR if this one is accepted).