cue: recursively marshal JSON values directly

cue.Value implements json.Marshaler, so any call of json.Marshal
with a cue.Value is a roundabout way to just call cue.Value.MarshalJSON.
Moreover, we can call the internal API directly, which gives us
more flexibility such as reusing buffers later on.

                            │     old      │                new                 │
                            │    sec/op    │   sec/op     vs base               │
    LargeValueMarshalJSON-8   106.23m ± 1%   12.19m ± 1%  -88.52% (p=0.002 n=6)

                            │     old      │                 new                 │
                            │     B/op     │     B/op      vs base               │
    LargeValueMarshalJSON-8   48.24Mi ± 8%   23.51Mi ± 0%  -51.27% (p=0.002 n=6)

                            │     old      │                new                 │
                            │  allocs/op   │  allocs/op   vs base               │
    LargeValueMarshalJSON-8   106.17k ± 2%   78.36k ± 0%  -26.19% (p=0.002 n=6)

Updates #2470.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I1b0713489a5e888ab997389257772d2249d4389b
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1201807
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mvdan

cue/types.go

@@ -154,7 +154,7 @@ func (o *structValue) marshalJSON() (b []byte, err error) {
 		}
 		b = append(b, s...)
 		b = append(b, ':')
-		bb, err := internaljson.Marshal(v)
+		bb, err := v.marshalJSON()
 		if err != nil {
 			return nil, err
 		}
@@ -298,7 +298,7 @@ func marshalList(l *Iterator) (b []byte, err error) {
 	b = append(b, '[')
 	if l.Next() {
 		for i := 0; ; i++ {
-			x, err := internaljson.Marshal(l.Value())
+			x, err := l.Value().marshalJSON()
 			if err != nil {
 				return nil, err
 			}