errors: optimize *joinError's Error

Remove TODOs, stale comment.

src/errors/join.go

@@ -44,24 +44,17 @@ type joinError struct {
 func (e *joinError) Error() string {
 	// Since Join returns nil if every value in errs is nil,
 	// e.errs cannot be empty.
-	// TODO: get rid of case 0
-	switch len(e.errs) {
-	case 0: // Impossible but handle.
-		return "<nil>"
-	case 1:
+	if len(e.errs) == 1 {
 		return e.errs[0].Error()
 	}
 
-	// To avoid doubling the number of Error calls, skip preallocating
-	// the slice on overflow and let the runtime handle the panic.
 	b := []byte(e.errs[0].Error())
 	for _, err := range e.errs[1:] {
 		b = append(b, '\n')
 		b = append(b, err.Error()...)
 	}
 	// At this point, b has at least one byte '\n'.
-	// TODO: replace with unsafe.String(&b[0], len(b))
-	return unsafe.String(unsafe.SliceData(b), len(b))
+	return unsafe.String(&b[0], len(b))
 }
 
 func (e *joinError) Unwrap() []error {