It is a fairly costly operation:
```
benchmark old ns/op new ns/op delta
BenchmarkScratchGet-4 109 31.6 -71.01%
benchmark old allocs new allocs delta
BenchmarkScratchGet-4 0 0 +0.00%
benchmark old bytes new bytes delta
BenchmarkScratchGet-4 0 0 +0.00%
´´´
The variable scope in the Go templates makes it hard, if possible at all, to write templates with counter variables or similar state.
This commit fixes that by adding a writable context to Node, backed by a map: Scratch.
This context has three methods, Get, Set and Add. The Add is tailored for counter variables, but can be used for any built-in numeric values or strings.
