syntax: Add support for trait bounds on procs

[alexcrichton]

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.

[alexcrichton]

Interestingly, this syntax leads to an ambiguity:

proc(proc:Send())

Is this parsed as a variable named proc with a type of Send(), or a procedure with a Send bound? Did we have another intended syntax for specifying bounds on procedures?

[huonw]

Isn't proc a keyword, and so not legal as a variable name?

[huonw]

I talked to @alexcrichton on IRC and he pointed out that the problem basically let x: |a: int| let x: |int| are both valid and equivalent, i.e. the former is just the latter with the argument named.

This means that, when parsing the internals of |proc:Send... (or proc(proc:Send) the parser sees the ident token proc and the : and assumes it's parsing a named argument even though the proc keyword is invalid to use as an argument name.

i.e. it's not necessarily an ambiguity, so much as an edgecase the compiler handles incorrectly. It seems a similar thing happens with let x: |&Trait:Send|, it parses &Trait as a pattern and gets confused by the use of the bare trait Send.