bytes: make all readOp constants actually typed

This is a regression introduced in golang.org/cl/28817. That change got
rid of the iota, which meant that the type was no longer applied to all
the constant names.

Re-add the iota starting at -1, simplifying the code and adding the
types once more.

Change-Id: I38bd0e04f8d298196bccd33651e29f5011401a8d
Reviewed-on: https://go-review.googlesource.com/78617
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/bytes/buffer.go

@@ -31,12 +31,12 @@ type Buffer struct {
 type readOp int8
 
 const (
-	opRead      readOp = -1 // Any other read operation.
-	opInvalid          = 0  // Non-read operation.
-	opReadRune1        = 1  // Read rune of size 1.
-	opReadRune2        = 2  // Read rune of size 2.
-	opReadRune3        = 3  // Read rune of size 3.
-	opReadRune4        = 4  // Read rune of size 4.
+	opRead      readOp = iota - 1 // Any other read operation.
+	opInvalid                     // Non-read operation.
+	opReadRune1                   // Read rune of size 1.
+	opReadRune2                   // Read rune of size 2.
+	opReadRune3                   // Read rune of size 3.
+	opReadRune4                   // Read rune of size 4.
 )
 
 // ErrTooLarge is passed to panic if memory cannot be allocated to store data in a buffer.