Revert "cue: remove Merge"

This reverts https://cuelang.org/cl/1193719.

We broke `cue cmd ls ./...` in cuelang.org's k8s tutorial.
We will add a regression test and attempt these changes a second time.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Ie2e5ee2209cb5d2940d82ddf535125c815825faa
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193746
Reviewed-by: Paul Jolly <paul@myitcv.io>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

cue/instance.go

@@ -217,6 +217,29 @@ func (inst *hiddenInstance) Eval(expr ast.Expr) Value {
 	return v.Context().BuildExpr(expr, Scope(v), InferBuiltins(true))
 }
 
+// DO NOT USE.
+//
+// Deprecated: do not use.
+func Merge(inst ...*Instance) *Instance {
+	v := &adt.Vertex{}
+
+	i := inst[0]
+	ctx := newContext(i.index)
+
+	// TODO: interesting test: use actual unification and then on K8s corpus.
+
+	for _, i := range inst {
+		w := i.Value()
+		v.AddConjunct(adt.MakeRootConjunct(nil, w.v.ToDataAll(ctx)))
+	}
+	v.Finalize(ctx)
+
+	p := addInst(i.index, &Instance{
+		root: v,
+	})
+	return p
+}
+
 // Build creates a new instance from the build instances, allowing unbound
 // identifier to bind to the top-level field in inst. The top-level fields in
 // inst take precedence over predeclared identifier and builtin functions.