internal/bytealg: reimplement bytealg in pure Go

Previously, we implemented individual bytealg functions via linknaming, and had to update them every once in a while when we hit linker errors.
Instead, this change reimplements the bytealg package in pure Go.
If something is missing, it will cause a compiler error rather than a linker error.
This is easier to test and maintain.

src/runtime/string.go

@@ -204,15 +204,3 @@ func decodeUTF8(s string, index uintptr) (rune, uintptr) {
 		return 0xfffd, 1
 	}
 }
-
-// indexByteString returns the index of the first instance of c in s, or -1 if c
-// is not present in s.
-//go:linkname indexByteString internal/bytealg.IndexByteString
-func indexByteString(s string, c byte) int {
-	for i := 0; i < len(s); i++ {
-		if s[i] == c {
-			return i
-		}
-	}
-	return -1
-}