Auto merge of #44049 - alexcrichton:nounwind-allocators, r=BurntSushi

Auto merge of #44049 - alexcrichton:nounwind-allocators, r=BurntSushi

std: Mark allocation functions as nounwind

This commit flags all allocation-related functions in liballoc as "this can't
unwind" which should largely resolve the size-related issues found on #42808.
The documentation on the trait was updated with such a restriction (they can't
panic) as well as some other words about the relative instability about
implementing a bullet-proof allocator.

Closes #42808