PGE: return values in embedded closures

I've just applied a change (r11921) to PGE that allows
embedded closures in rules to designate a "return value" for
a match (recently added to S05).

Essentially this means that a rule such as

    /:lang(PIR) foo {{ .return "bar" }} /

will match a string containing "foo" but gives the returned match
object a value of "bar".  

We can also do things such as

    /:lang(PIR) \d+ {{             # return 200 more than matched
       $I0 = match
       $I0 += 200
       .return ($I0) }} /  

    /:lang(PIR) <moose> {{
       $P0 = match["moose"]        # get subpattern match
       $P1 = "Moose_new"($P0)      # create a new Moose object from it
       .return ($P0) }} /          # and return that as the Match value
                           
To obtain the return value of a Match object, use the value() method:

       match = rule(string)        # match string against rule
       $P0 = match.value()

If a match object doesn't have an explicit return value, then
its value is the substring that it matched.

There are some small examples in t/compilers/pge/p6rules/closure.t .
Comments, questions and tests welcomed.

Pm