`hex.EncodeToString` has better performance than `fmt.Sprintf("%x",
[]byte)`, we should use it as much as possible.
I'm not an extreme fan of performance, so I think there are some
exceptions:
- `fmt.Sprintf("%x", func(...)[N]byte())`
- We can't slice the function return value directly, and it's not worth
adding lines.
```diff
func A()[20]byte { ... }
- a := fmt.Sprintf("%x", A())
- a := hex.EncodeToString(A()[:]) // invalid
+ tmp := A()
+ a := hex.EncodeToString(tmp[:])
```
- `fmt.Sprintf("%X", []byte)`
- `strings.ToUpper(hex.EncodeToString(bytes))` has even worse
performance.
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