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/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)
 }