Auto merge of #123080 - Jules-Bertholet:mut-ref-mut, r=Nadrieril

Match ergonomics 2024: implement mutable by-reference bindings

Implements the mutable by-reference bindings portion of match ergonomics 2024 (#123076), with the `mut ref`/`mut ref mut` syntax, under feature gate `mut_ref`.

r? `@Nadrieril`

`@rustbot` label A-patterns A-edition-2024

Author: bors