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/bytes.go

@@ -1,11 +0,0 @@
-package runtime
-
-//go:linkname indexBytePortable internal/bytealg.IndexByte
-func indexBytePortable(s []byte, c byte) int {
-	for i, b := range s {
-		if b == c {
-			return i
-		}
-	}
-	return -1
-}

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
-}

src/runtime/string_count.go

@@ -1,23 +0,0 @@
-// +build amd64 arm,go1.13 arm64 ppc64le ppc64 s390x
-
-package runtime
-
-// This file implements the string counting functions used by the strings
-// package, for example. It must be reimplemented here as a replacement for the
-// Go stdlib asm implementations, but only when the asm implementations are used
-// (this varies by Go version).
-// Track this file for updates:
-// https://github.com/golang/go/blob/master/src/internal/bytealg/count_native.go
-
-// countString copies the implementation from
-// https://github.com/golang/go/blob/67f181bfd84dfd5942fe9a29d8a20c9ce5eb2fea/src/internal/bytealg/count_generic.go#L1
-//go:linkname countString internal/bytealg.CountString
-func countString(s string, c byte) int {
-	n := 0
-	for i := 0; i < len(s); i++ {
-		if s[i] == c {
-			n++
-		}
-	}
-	return n
-}

src/runtime/strings_go111.go

@@ -2,11 +2,13 @@
 
 package runtime
 
+import "internal/bytealg"
+
 // indexByte provides compatibility with Go 1.11.
 // See the following:
 // https://github.com/tinygo-org/tinygo/issues/351
 // https://github.com/golang/go/commit/ad4a58e31501bce5de2aad90a620eaecdc1eecb8
 //go:linkname indexByte strings.IndexByte
 func indexByte(s string, c byte) int {
-	return indexByteString(s, c)
+	return bytealg.IndexByteString(s, c)
 }