RFC: Pattern Guards with Bind-By-Move
Add another rejected example.
Add a mention of a feature gate

In the Rust meeting it was decided that this should go in behind a
feature gate. I arbitrarily chose the `bind_by_move_pattern_guards`
name.
Merge remote-tracking branch 'zwarich/pattern-guards-with-bind-by-move'
Pattern guards with bind-by-move is RFC 36