performance: reduce the number of fills

- Remove unnecessary Fill() in Export()
- Change `set()` and the way we store outputs so we don't fill
  intermediaries as much
- WIP: Scan the tree only once. Changed LocalDirs to use cueflow rather than
  doing our own Walk. In a follow up we should use the same flow
  instance.

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

dagger/compiler/compiler.go

@@ -17,7 +17,7 @@ func Compile(name string, src interface{}) (*Value, error) {
 	return DefaultCompiler.Compile(name, src)
 }
 
-func EmptyStruct() (*Value, error) {
+func EmptyStruct() *Value {
 	return DefaultCompiler.EmptyStruct()
 }
 
@@ -63,8 +63,12 @@ func (c *Compiler) Cue() *cue.Runtime {
 }
 
 // Compile an empty struct
-func (c *Compiler) EmptyStruct() (*Value, error) {
-	return c.Compile("", "")
+func (c *Compiler) EmptyStruct() *Value {
+	empty, err := c.Compile("", "")
+	if err != nil {
+		panic(err)
+	}
+	return empty
 }
 
 func (c *Compiler) Compile(name string, src interface{}) (*Value, error) {

dagger/compiler/compiler_test.go

@@ -38,14 +38,6 @@ func TestDefNotExist(t *testing.T) {
 	}
 }
 
-func TestSimple(t *testing.T) {
-	c := &Compiler{}
-	_, err := c.EmptyStruct()
-	if err != nil {
-		t.Fatal(err)
-	}
-}
-
 func TestJSON(t *testing.T) {
 	c := &Compiler{}
 	v, err := c.Compile("", `foo: hello: "world"`)

dagger/compiler/value.go

@@ -67,6 +67,11 @@ func (v *Value) Len() cue.Value {
 	return v.val.Len()
 }
 
+// Proxy function to the underlying cue.Value
+func (v *Value) Kind() cue.Kind {
+	return v.val.Kind()
+}
+
 // Proxy function to the underlying cue.Value
 func (v *Value) Fields() (*cue.Iterator, error) {
 	return v.val.Fields()