Adds M::CurriedRoleHow

Mainly for the sake of illustration. Text copied from @jnthn comment
at the beginning of the source code. Refs #1306

doc/Type/Metamodel/CurriedRoleHOW.pod6

@@ -0,0 +1,35 @@
+=begin pod
+
+=TITLE role Metamodel::CurriedRoleHOW
+
+=SUBTITLE Support for parametrized roles that have not been instantiated
+
+    class Metamodel::CurriedRoleHOW
+        does Metamodel::Naming
+        does Metamodel::TypePretense
+        does Metamodel::RolePunning {}
+
+Sometimes, we see references to roles that provide parameters but
+do not fully resolve them. For example, in:
+
+=for code :preamble<role R[::Type] {}; class Type {}>
+class C does R[Type] { }
+
+We need to represent C<R[T]>, but we cannot yet fully specialize the
+role because we don't have the first parameter at hand. We may also
+run into the issue where we have things like:
+
+=for code :preamble<role R[::T] {}; class T {}; my $x>
+sub foo(R[T] $x) { ... }
+if $x ~~ R[T] { ... }
+
+Where we clearly want to talk about a partial parameterization of a
+role and actually want to do so in a way distinct from a particular
+instantiation of it. This meta-object represents those "partial types"
+as both a way to curry on your way to a full specialization, but also
+as a way to do type-checking or punning.
+
+I<Note>: As most of the Metamodel classes, this class is here mainly for
+illustration purposes and it's not intended for the final user.
+
+=end pod