Simple collect optimization

[curlup]

Was

$ go test -bench . -benchtime 4s
BenchmarkTieKeys-4        	 1000000	      8525 ns/op
BenchmarkPrefixSearch-4   	   10000	    627035 ns/op
BenchmarkFuzzySearch-4    	    1000	   8602394 ns/op
BenchmarkBuildTree-4      	      30	 175859534 ns/op
PASS
ok  	trie	31.224s

After fix

$ go test -bench . -benchtime 4s
BenchmarkTieKeys-4        	 1000000	      6502 ns/op
BenchmarkPrefixSearch-4   	   10000	    403476 ns/op
BenchmarkFuzzySearch-4    	    1000	   6794919 ns/op
BenchmarkBuildTree-4      	      30	 173354546 ns/op
PASS
ok  	trie	24.806s