Prefer strconv over fmt

When converting primitives to/from strings, strconv is faster than fmt.

Bad Good

Bad	Good
`for i := 0; i < b.N; i++ { s := fmt.Sprint(rand.Int()) }`	`for i := 0; i < b.N; i++ { s := strconv.Itoa(rand.Int()) }`
`BenchmarkFmtSprint-4 143 ns/op 2 allocs/op`	`BenchmarkStrconv-4 64.2 ns/op 1 allocs/op`

for i := 0; i < b.N; i++ {
  s := fmt.Sprint(rand.Int())
}

for i := 0; i < b.N; i++ {
  s := strconv.Itoa(rand.Int())
}

BenchmarkFmtSprint-4    143 ns/op    2 allocs/op

BenchmarkStrconv-4    64.2 ns/op    1 allocs/op