Optimize compressionLenHelper

Back with another optimization :)

`compressionLenHelper` was making quite a few string concatenations, so I opted to slice up the original string instead of creating new ones. It improved the performance quite a bit.

Relevant benchmarks:

```
github.com/miekg/dns
---
BenchmarkMsgLen	10000000	      2371 ns/op	     591 B/op	      12 allocs/op


github.com/millerkil/dns branch:opt-compress-help
---
BenchmarkMsgLen	20000000	      1208 ns/op	     294 B/op	       3 allocs/op
```