Adds ParametricRoleGroupHOW

This part becomes a bit more complete, and allows us to understand the
inner workings (in fact, the *high-order-work* of Perl 6). Although
they are not (so much) intended for final user, they have been used,
for instance, in https://github.com/vrurg/Perl6-OO-Plugin

This closes #1306. Although there might be a few other metamodel
classes that have not been documented, they are probably deeper down
in the implementation and will never be seen by the user. There's no
request to use them, anyway.

doc/Type/Metamodel/ParametricRoleGroupHOW.pod6

@@ -0,0 +1,32 @@
+=begin pod
+
+=TITLE role Metamodel::ParametricRoleGroupHOW
+
+=SUBTITLE Represents a group of roles with different parametrizations
+
+    class Metamodel::ParametricRoleGroupHOW
+        does Metamodel::Naming
+        does Metamodel::Documenting
+        does Metamodel::Stashing
+        does Metamodel::TypePretense
+        does Metamodel::RolePunning
+        does Metamodel::BoolificationProtocol {}
+
+A C<ParametricRoleGroupHOW> groups a set of C<ParametricRoleHOW>, every one of them representing a single role declaration with their own parameter sets.
+
+=for code
+(role Zape[::T] {}).HOW.say; # OUTPUT: «Perl6::Metamodel::ParametricRoleHOW.new␤»
+Zape.HOW.say ; # OUTPUT: «Perl6::Metamodel::ParametricRoleGroupHOW.new␤»
+
+C<ParametricRoleHOW>s need to be added to this kind of group:
+
+=for code
+my \zape := Metamodel::ParametricRoleGroupHOW.new_type( name => "zape");
+my \zipi := Metamodel::ParametricRoleHOW.new_type( name => "zipi", group => zape);
+say zipi.HOW; # OUTPUT: «Perl6::Metamodel::ParametricRoleHOW.new␤»
+
+
+I<Note>: As most of the C<Metamodel> classes, this class is here mainly for
+illustration purposes and it's not intended for the final user to instantiate.
+
+=end pod