Reduce allocations when reusing a Builder.

Add the function `Reset` to the Builder, which facilitates reuse of the
underlying byte slice.

tests/GoTest.sh

@@ -43,7 +43,7 @@ GOPATH=${go_path} go test flatbuffers_test \
                      --test.coverpkg=github.com/google/flatbuffers/go \
                      --cpp_data=${test_dir}/monsterdata_test.mon \
                      --out_data=${test_dir}/monsterdata_go_wire.mon \
-                     --test.bench=Build \
+                     --test.bench=. \
                      --test.benchtime=3s \
                      --fuzz=true \
                      --fuzz_fields=4 \

tests/go_test.go

@@ -1254,9 +1254,11 @@ func BenchmarkBuildGold(b *testing.B) {
 	reuse_fred := []byte("Fred")
 
 	b.SetBytes(bytes_length)
+	bldr := flatbuffers.NewBuilder(512)
 	b.ReportAllocs()
 	for i := 0; i < b.N; i++ {
-		bldr := flatbuffers.NewBuilder(0)
+		bldr.Reset()
+
 		str := bldr.CreateByteString(reuse_str)
 		test1 := bldr.CreateByteString(reuse_test1)
 		test2 := bldr.CreateByteString(reuse_test2)