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/polydoubleinclude/main.mcl

@@ -0,0 +1,10 @@
+import "fmt"
+
+# note that the class can have two separate types for $b
+include c1("t1", "hello") # len is 5
+include c1("t2", [13, 42, 0, -37,]) # len is 4
+class c1($a, $b) {
+	test $a {
+		anotherstr => fmt.printf("len is: %d", len($b)),
+	}
+}