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/gapi/gapi.go

@@ -268,6 +268,10 @@ func (obj *GAPI) Cli(info *gapi.Info) (*gapi.Deploy, error) {
 	if name := args.UnifySolver; name != nil && *name != "" {
 		unificationStrategy[unification.StrategyNameKey] = *name
 	}
+	if len(args.UnifyOptimizations) > 0 {
+		// TODO: use a query string parser instead?
+		unificationStrategy[unification.StrategyOptimizationsKey] = strings.Join(args.UnifyOptimizations, ",")
+	}
 
 	if !args.SkipUnify {
 		// apply type unification