bytes: don't use an iota for the readOp constants

As per the comments in golang.org/cl/78617. Also leaving a comment here,
to make sure noone else thinks to re-introduce the iota like I did.

Change-Id: I2a2275998b81896eaa0e9d5ee0197661ebe84acf
Reviewed-on: https://go-review.googlesource.com/78676
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

@@ -30,13 +30,15 @@ type Buffer struct {
 // converted to int they correspond to the rune size that was read.
 type readOp int8
 
+// Don't use iota for these, as the values need to correspond with the
+// names and comments, which is easier to see when being explicit.
 const (
-	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.
+	opRead      readOp = -1 // Any other read operation.
+	opInvalid   readOp = 0  // Non-read operation.
+	opReadRune1 readOp = 1  // Read rune of size 1.
+	opReadRune2 readOp = 2  // Read rune of size 2.
+	opReadRune3 readOp = 3  // Read rune of size 3.
+	opReadRune4 readOp = 4  // Read rune of size 4.
 )
 
 // ErrTooLarge is passed to panic if memory cannot be allocated to store data in a buffer.