Also: Be explicit what operator means with respect to conversion types.

The parenthesis requirement is a language change. At the moment,
literal function types in conversions that cannot possibly be
followed by a '(' don't need parentheses. For instance:

        func(int)int(x)  ->  same as (func(int)int)(x)
        func()()(x)      ->  same as (func())(x)

but:

        func(int)(x)	 ->  could be func(int)x {...}

Fixes #4109.

R=rsc, r, iant, ken, iant
CC=golang-dev
https://golang.org/cl/6584065