Work around pointer aliasing issue in Vec::extend_from_slice, extend_with_element

Work around pointer aliasing issue in Vec::extend_from_slice, extend_with_element

Due to missing noalias annotations for &mut T in general (issue #31681),
in larger programs extend_from_slice and extend_with_element may both
compile very poorly. What is observed is that the .set_len() calls are
not lifted out of the loop, even for `Vec<u8>`.

Use a local length variable for the Vec length instead, and use a scope
guard to write this value back to self.len when the scope ends or on
panic. Then the alias analysis is easy.

This affects extend_from_slice, extend_with_element, the vec![x; n]
macro, Write impls for Vec<u8>, BufWriter, etc (but may / may not
have triggered since inlining can be enough for the compiler to get it right).