Use NextLabel in compressionLenHelper

[tmthrgd]

This avoids the allocation of Split and should have slightly better performance.

This was pulled out of #818 and is reflected in the benchmarks there.

[codecov-io]

Codecov Report

Merging #826 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #826      +/-   ##
==========================================
+ Coverage   57.53%   57.57%   +0.03%     
==========================================
  Files          42       42              
  Lines       10825    10823       -2     
==========================================
+ Hits         6228     6231       +3     
+ Misses       3517     3513       -4     
+ Partials     1080     1079       -1

Impacted Files	Coverage Δ
msg.go	78.31% <100%> (+0.57%)	⬆️
server.go	68.68% <0%> (+0.24%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1c9c9bf...b0597cc. Read the comment docs.

[tmthrgd]

Hold off on this for now. I'm working on replacing compressionLenHelper entirely and this will only cause difficulty merging.