Reject method declarations on interface types

Interfaces are abstract and cannot be receivers. The compiler was
silently accepting the declaration and storing the method as an
unreachable global — the runtime would never dispatch to it because
interface values are always their concrete underlying type.

Add a registry check in methodDecl that returns MethodOnInterface
before the function body is compiled.

Author: ganehag

examples/spec/fail/111_method_on_interface.err

@@ -0,0 +1 @@
+MethodOnInterface

examples/spec/fail/111_method_on_interface.gengo

@@ -0,0 +1,2 @@
+type Shape interface { area() float }
+func (s Shape) fval() float { return 0.0 }

src/lang/compiler.zig

@@ -1066,6 +1066,7 @@ pub const Compiler = struct {
         if (self.cur.typ != .ident) return error.UnexpectedToken;
         const recv_type = self.cur.src;
         self.advance();
+        if (self.registry.hasInterfaceType(recv_type)) return error.MethodOnInterface;
         try self.consume(.rparen);
         if (self.cur.typ != .ident) return error.UnexpectedToken;
         const method_name = self.cur.src;