consider removing `allow(unused_mut)` from core and std doctests

[lolbinarycat]

A lot of new rust users get confused about the relationship between mut bindings and &mut, and the doctests don't really help with this, with core alone having 21 doctests emitting this lint. most of these are mut bindings that hold a mutable reference.

[lolbinarycat]

The line that would need to be changed is the doc(test(attr(allow stack in the respective lib.rs files.