a few optimizations

[OneOfOne]

This a few minor optimizations I applied to my local fork, thought I'd share back.

the biggest one is findCaseInsensitivePath:

#before
BenchmarkTreeFindCaseInsensitivePath-32           121699              9786 ns/op             320 B/op         26 allocs/op
# after
BenchmarkTreeFindCaseInsensitivePath-32           123482              9771 ns/op             176 B/op         25 allocs/op