Allow ref and mut modifiers for short form field patterns

Previously, if you wanted to bind a field mutably or by ref, you had to
do something like Foo { x: ref mut x }. You can now just do
Foo { ref mut x }.

Closes #6137
auto merge of #10833 : sfackler/rust/mut-pat, r=brson

Previously, if you wanted to bind a field mutably or by ref, you had to
do something like Foo { x: ref mut x }. You can now just do
Foo { ref mut x }.

Closes #6137