Parse subsignatures

See SO6/Closure parameters.  For example:

    sub limited_grep(Int $count, &block:(Dog)), *@list) { ... }

These currently sort-of parse, but :(Dog) is considered a *trait*, and
that doesn't sit right with something in the compiler, so it results in
this error:

    Cannot invoke this object (REPR: P6opaque, cs = 0)

src/Perl6/Grammar.nqp

@@ -2822,6 +2822,11 @@ grammar Perl6::Grammar is HLL::Grammar does STD {
           || <name=.decint> { $*W.throw($/, 'X::Syntax::Variable::Numeric', what => 'parameter') }
           || $<name>=[<[/!]>]
           ]?
+
+          [
+              <?{ ~$<sigil> eq '&' }>
+              ':(' ~ ')' <subsignature=.signature>
+          ]?
         ]
     }