I'm trying to a write a generic function that operates on either string | []byte. However, I'm unable to do so since the implementation of that function needs to depend on either utf8.DecodeRune or utf8.DecodeRuneInString, but I'm unable to express that as a simple expression without using a type switch.

I propose we add generic versions of:

func DecodeRune[Bytes []byte | string](p Bytes) (r rune, size int)
func DecodeLastRune[Bytes []byte | string](p Bytes) (r rune, size int)
func FullRune[Bytes []byte | string](p Bytes) bool
func RuneCount[Bytes []byte | string](p Bytes) int
func Valid[Bytes []byte | string](p Bytes) bool

It is unclear what the name should be since the simpler names are already taken by the non-generic variants.
Perhaps, we should have a v2 variant of utf8 that operates on either type.