70% faster when using a high number of channels
Old performance
```
$ go test ./state -bench=.
goos: linux
goarch: amd64
pkg: github.com/fluffle/goirc/state
cpu: AMD Ryzen Threadripper PRO 3945WX 12-Cores
BenchmarkNickSingleChan-24 15465226 76.35 ns/op
BenchmarkNickManyChan-24 5738 195721 ns/op
PASS
ok github.com/fluffle/goirc/state 2.433s
```
new performance
```
$ go test ./state -bench=.
goos: linux
goarch: amd64
pkg: github.com/fluffle/goirc/state
cpu: AMD Ryzen Threadripper PRO 3945WX 12-Cores
BenchmarkNickSingleChan-24 15022640 79.13 ns/op
BenchmarkNickManyChan-24 10000 115104 ns/op
PASS
ok github.com/fluffle/goirc/state 2.456s
```