lang: Add the NamedArgs interface

This lets you specify which args are being used in the general function
API, which can make code readability and debugability slightly better.

In an ideal world, we wouldn't need this at all, but I can't figure out
how to avoid it at the moment, so we'll include it for now, as it's
always easy to delete if we find a more elegant solution.

lang/interfaces/func.go

@@ -90,3 +90,14 @@ type PolyFunc interface {
 	// are all that should be needed or used after this point.)
 	Build(*types.Type) error // then, you can get argNames from Info()
 }
+
+// NamedArgsFunc is a function that uses non-standard function arg names. If you
+// don't implement this, then the argnames (if specified) must correspond to the
+// a, b, c...z, aa, ab...az, ba...bz, and so on sequence.
+type NamedArgsFunc interface {
+	Func // implement everything in Func but add the additional requirements
+
+	// ArgGen implements the arg name generator function. By default, we use
+	// the util.NumToAlpha function when this interface isn't implemented...
+	ArgGen(int) string
+}