Assigning to a `&mut T`

Decided

D-0044, settled by the language’s author: part of spec §18. Older text cites it as 18.44.

Writes the T it refers to, as in C++. A reference cannot be pointed elsewhere, so there was no other meaning to give it, and for &mut n in counts { n += 1 } needs it. Through a &T it is an error (§5.2)