output: Speed up layout calculations

```
BenchmarkLayout-4     4883          497           -89.82%

benchmark             old allocs     new allocs     delta
BenchmarkLayout-4     18             1              -94.44%

benchmark             old bytes     new bytes     delta
BenchmarkLayout-4     1624          32            -98.03%
```

output/layout_test.go

@@ -73,4 +73,15 @@ func TestLayout(t *testing.T) {
 			}
 		})
 	}
+
+}
+
+func BenchmarkLayout(b *testing.B) {
+	descriptor := LayoutDescriptor{Kind: "taxonomyTerm", Section: "categories"}
+	l := NewLayoutHandler(false)
+
+	for i := 0; i < b.N; i++ {
+		layouts := l.For(descriptor, "", HTMLType)
+		require.NotEmpty(b, layouts)
+	}
 }