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

src/liballoc/lib.rs

@@ -107,6 +107,7 @@
 #![feature(pattern)]
 #![feature(placement_in_syntax)]
 #![feature(placement_new_protocol)]
+#![feature(rustc_attrs)]
 #![feature(shared)]
 #![feature(slice_get_slice)]
 #![feature(slice_patterns)]