lang: unification: Improve type unification algorithm

The simple type unification algorithm suffered from some serious
performance and memory problems when used with certain code bases. This
adds some crucial optimizations that improve performance drastically.

lang/interpret_test/TestAstFunc1/doubleinclude/main.mcl

@@ -0,0 +1,8 @@
+include c1("t1")
+include c1("t2")
+class c1($a) {
+	test $a {
+		stringptr => $foo,
+	}
+}
+$foo = "hey"