lang: Add unification optimizations

This adds a unification optimizations API, and uses it to optimize the
embedded provisioner. With these turned on, type unification drops from
around 1m45s to 2.5s which is a 40x speedup.

lang/lang.go

@@ -242,6 +242,7 @@ func (obj *Lang) Init(ctx context.Context) error {
 
 	// apply type unification
 	logf := func(format string, v ...interface{}) {
+		// TODO: Remove the masked logger here when unification is clean!
 		if obj.Debug { // unification only has debug messages...
 			obj.Logf("unification: "+format, v...)
 		}