cue: unexport NewInstance from api

Change-Id: Ic3fa2bdac484fc2340e4cf88254255b69f360501
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2543
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/cue/build.go b/cue/build.go
index ae23393..c1a6b70 100644
--- a/cue/build.go
+++ b/cue/build.go
@@ -105,7 +105,7 @@
 // FromExpr creates an instance from an expression.
 // Any references must be resolved beforehand.
 func (r *Runtime) FromExpr(expr ast.Expr) (*Instance, error) {
-	i := r.index().NewInstance(nil)
+	i := r.index().newInstance(nil)
 	err := i.insertFile(&ast.File{
 		Decls: []ast.Decl{&ast.EmitDecl{Expr: expr}},
 	})
@@ -223,7 +223,7 @@
 		return inst
 	}
 	files := p.Files
-	inst := idx.NewInstance(p)
+	inst := idx.newInstance(p)
 	if inst.Err == nil {
 		// inst.instance.index.state = s
 		// inst.instance.inst = p
diff --git a/cue/instance.go b/cue/instance.go
index 8df4cda..44a2ccb 100644
--- a/cue/instance.go
+++ b/cue/instance.go
@@ -64,8 +64,8 @@
 	return imp
 }
 
-// NewInstance creates a new instance. Use Insert to populate the instance.
-func (x *index) NewInstance(p *build.Instance) *Instance {
+// newInstance creates a new instance. Use Insert to populate the instance.
+func (x *index) newInstance(p *build.Instance) *Instance {
 	st := &structLit{baseValue: baseValue{nil}}
 	i := x.addInst(&Instance{
 		rootStruct: st,
@@ -224,7 +224,7 @@
 
 	idx := inst.index
 
-	i := idx.NewInstance(p)
+	i := idx.newInstance(p)
 	if i.Err != nil {
 		return i
 	}
diff --git a/cue/resolve_test.go b/cue/resolve_test.go
index 193a6ef..6125c7a 100644
--- a/cue/resolve_test.go
+++ b/cue/resolve_test.go
@@ -43,7 +43,7 @@
 	inst, err := r.Parse("test", body)
 
 	if err != nil {
-		x := newIndex().NewInstance(nil)
+		x := newIndex().newInstance(nil)
 		ctx := x.newContext()
 		return ctx, x, err
 	}