syntax: Add support for trait bounds on procs

This is needed to make progress on #10296 as the default bounds will no longer
include Send. I believe that this was the originally intended syntax for procs,
and it just hasn't been necessary up until now.
auto merge of #12774 : alexcrichton/rust/proc-bounds, r=pcwalton

This is needed to make progress on #10296 as the default bounds will no longer
include Send. I believe that this was the originally intended syntax for procs,
and it just hasn't been necessary up until now.