geojson: use nocopyRawMessage for feature collection unmarshal

no speed improvement but lots of bytes saved

benchmark                             old bytes     new bytes     delta
BenchmarkFeatureUnmarshalJSON-12      941182        828250        -12.00%

geojson/feature.go

@@ -57,7 +57,7 @@ func (f Feature) MarshalJSON() ([]byte, error) {
 // Alternately one can call json.Unmarshal(f) directly for the same result.
 func UnmarshalFeature(data []byte) (*Feature, error) {
 	f := &Feature{}
-	err := json.Unmarshal(data, f)
+	err := f.UnmarshalJSON(data)
 	if err != nil {
 		return nil, err
 	}

geojson/feature_collection.go

@@ -69,7 +69,7 @@ func (fc FeatureCollection) MarshalJSON() ([]byte, error) {
 // UnmarshalJSON decodes the data into a GeoJSON feature collection.
 // Extra/foreign members will be put into the `ExtraMembers` attribute.
 func (fc *FeatureCollection) UnmarshalJSON(data []byte) error {
-	tmp := make(map[string]json.RawMessage, 4)
+	tmp := make(map[string]nocopyRawMessage, 4)
 
 	err := json.Unmarshal(data, &tmp)
 	if err != nil {