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/unification_test.go

@@ -819,7 +819,13 @@ func TestUnification1(t *testing.T) {
 			logf := func(format string, v ...interface{}) {
 				t.Logf(fmt.Sprintf("test #%d", index)+": unification: "+format, v...)
 			}
-			err := unification.Unify(ast, unification.SimpleInvariantSolverLogger(logf))
+			unifier := &unification.Unifier{
+				AST:    ast,
+				Solver: unification.SimpleInvariantSolverLogger(logf),
+				Debug:  testing.Verbose(),
+				Logf:   logf,
+			}
+			err := unifier.Unify()
 
 			// TODO: print out the AST's so that we can see the types
 			t.Logf("\n\ntest #%d: AST (after): %+v\n", index, ast)