all: replaces []byte(fmt.Sprintf(...)) by fmt.Appendf(nil, ...)

[islishude]

I just had a benchmark, the fmt.Appendf is a little faster and it uses less memory.

import "fmt"

func FormatByte1(n int) []byte {
	return []byte(fmt.Sprintf("key-%04d", n))
}

func FormatByte2(n int) []byte {
	return fmt.Appendf(nil, "key-%04d", n)
}

func BenchmarkFormatByte(b *testing.B) {
	const n = 1024

	b.Run("FormatByte1", func(b *testing.B) {
		for b.Loop() {
			FormatByte1(n)
		}
	})

	b.Run("FormatByte2", func(b *testing.B) {
		for b.Loop() {
			FormatByte2(n)
		}
	})
}

BenchmarkFormatByte
BenchmarkFormatByte/FormatByte1
BenchmarkFormatByte/FormatByte1-16         	17977934	        66.30 ns/op	      24 B/op	       3 allocs/op
BenchmarkFormatByte/FormatByte2
BenchmarkFormatByte/FormatByte2-16         	20140873	        59.02 ns/op	      16 B/op	       2 allocs/op
PASS
coverage: 100.0% of statements