cue: return an ast.Expr instead of Instance

Fixes cuelang/cue#21.

Change-Id: I214a7bf96db69d02deedc6db06c3b5945298d9dc
diff --git a/cue/ast.go b/cue/ast.go
index e1303ea..9dbed65 100644
--- a/cue/ast.go
+++ b/cue/ast.go
@@ -85,6 +85,10 @@
 
 func newVisitor(idx *index, inst *build.Instance, obj, resolveRoot *structLit) *astVisitor {
 	ctx := idx.newContext()
+	return newVisitorCtx(ctx, inst, obj, resolveRoot)
+}
+
+func newVisitorCtx(ctx *context, inst *build.Instance, obj, resolveRoot *structLit) *astVisitor {
 	v := &astVisitor{
 		object: obj,
 	}