Add a newScratch template func

Fixes #4685

tpl/collections/collections.go

@@ -23,6 +23,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/common/types"
 	"github.com/gohugoio/hugo/deps"
 	"github.com/gohugoio/hugo/helpers"
@@ -650,3 +651,9 @@ func (ns *Namespace) Uniq(l interface{}) (interface{}, error) {
 func (ns *Namespace) KeyVals(key interface{}, vals ...interface{}) (types.KeyValues, error) {
 	return types.KeyValues{Key: key, Values: vals}, nil
 }
+
+// NewScratch creates a new Scratch which can be used to store values in a
+// thread safe way.
+func (ns *Namespace) NewScratch() *maps.Scratch {
+	return maps.NewScratch()
+}

tpl/collections/init.go

@@ -144,6 +144,14 @@ func init() {
 				{`{{ seq 3 }}`, `[1 2 3]`},
 			},
 		)
+
+		ns.AddMethodMapping(ctx.NewScratch,
+			[]string{"newScratch"},
+			[][2]string{
+				{`{{ $scratch := newScratch }}{{ $scratch.Add "b" 2 }}{{ $scratch.Add "b" 2 }}{{ $scratch.Get "b" }}`, `4`},
+			},
+		)
+
 		ns.AddMethodMapping(ctx.Uniq,
 			[]string{"uniq"},
 			[][2]string{