Optimize collect

[curlup]

Was

$ go test -bench . -benchtime 4s -benchmem
TieKeys-4            1000000          8335 ns/op         847 B/op         67 allocs/op
PrefixSearch-4         10000        617542 ns/op       71653 B/op       6287 allocs/op
FuzzySearch-4           1000       8789504 ns/op      793950 B/op      52051 allocs/op
BuildTree-4               30     172820241 ns/op    64263468 B/op     908713 allocs/op
PASS
ok      _trie   30.907s

And now its

$ go test -bench . -benchtime 4s -benchmem
TieKeys-4            1000000          4583 ns/op         566 B/op          7 allocs/op
PrefixSearch-4         20000        219790 ns/op       33746 B/op         16 allocs/op
FuzzySearch-4           1000       5759077 ns/op      479819 B/op       5065 allocs/op
BuildTree-4               20     240212152 ns/op    78269795 B/op    1347616 allocs/op
PASS
ok      _trie   24.888s

It's better than #16, but there's additional memory footprint and time to build trie.

[curlup]

I've improved my result but still building trie takes more time/ops/mem than current impl

$ go test -bench . -benchtime 4s -benchmem
TieKeys-4            1000000          4616 ns/op         565 B/op          7 allocs/op
PrefixSearch-4         30000        217131 ns/op       33710 B/op         16 allocs/op
FuzzySearch-4           1000       5641379 ns/op      480559 B/op       5065 allocs/op
BuildTree-4               30     188456008 ns/op    70782355 B/op    1011021 allocs/op
PASS

[derekparker]

Merged via another PR.