PGE now supports grammars, built-in rules

[Patrick R. Michaud]

I've just checked in changes to PGE that enable it to support
grammars, as well as some more built-in rules (<alpha>,
<digit>, <alnum>, <ident>, etc.).  To create a new grammar,
just create a subclass of "PGE::Rule" and install new rules into
the new grammar's namespace:

    .sub main @MAIN
        load_bytecode "PGE.pbc"
        .local pmc p6rule
        p6rule = find_global "PGE", "p6rule"
    
        $P0 = getclass "PGE::Rule"
        $P1 = subclass $P0, "MyClass"
    
        # create a custom MyClass::alpha rule
        $P0 = p6rule("<[abcde]>")
        store_global "MyClass", "alpha", $P0
    
        $P0 = p6rule("^ <MyClass::ident>")
        $P1 = $P0("ab23")                      # matches "ab23"
        $I0 = $P1.__get_bool()
        print $I0
        print "\n"
    
        $P1 = $P0("zz23")                      # doesn't match zz23
        $I0 = $P1.__get_bool()
        print $I0
        print "\n"
    .end
  
I've added some more tests for the above, but many many more 
are needed.  If anyone wants to request (or better yet, submit
patches for) other of the default built-in rules, I'll be glad
to add them.
 
Pm