Optimize allocations in GetForId() about namegen HOT 1 CLOSED

BEFORE

Anands-MBP:namegen anand$ go test -bench=. -benchmem
goos: darwin
goarch: amd64
pkg: github.com/anandvarma/namegen
cpu: Intel(R) Core(TM) i5-4278U CPU @ 2.60GHz
BenchmarkNew-4 1000000000 0.3528 ns/op 0 B/op 0 allocs/op
BenchmarkGet-4 4428757 256.4 ns/op 65 B/op 3 allocs/op
BenchmarkGetForId-4 5436145 217.3 ns/op 64 B/op 3 allocs/op
BenchmarkGetWithPostfix-4 2684820 440.3 ns/op 162 B/op 5 allocs/op
BenchmarkGetPostfixId-4 17676880 65.42 ns/op 24 B/op 1 allocs/op
PASS

AFTER

Anands-MBP:namegen anand$ go test -bench=. -benchmem
goos: darwin
goarch: amd64
pkg: github.com/anandvarma/namegen
cpu: Intel(R) Core(TM) i5-4278U CPU @ 2.60GHz
BenchmarkNew-4 1000000000 0.3441 ns/op 0 B/op 0 allocs/op
BenchmarkGet-4 7847606 156.4 ns/op 27 B/op 1 allocs/op
BenchmarkGetWithId-4 12039793 125.4 ns/op 48 B/op 2 allocs/op
BenchmarkGetWithPostfix-4 4579980 259.4 ns/op 79 B/op 3 allocs/op
BenchmarkGetPostfixId-4 17482552 65.07 ns/op 24 B/op 1 allocs/op
PASS
ok github.com/anandvarma/namegen 7.189s

from namegen.