Including the type of a constant improves discoverability because the
constant is listed next to the type in the godocs.
Before:
```
$ go doc -all . WalkStatus
type WalkStatus int
WalkStatus represents a current status of the Walk function.
```
After:
```shell
$ go doc -all . WalkStatus
type WalkStatus int
WalkStatus represents a current status of the Walk function.
const (
// WalkStop indicates no more walking needed.
WalkStop WalkStatus = iota + 1
// WalkSkipChildren indicates that Walk wont walk on children of current
// node.
WalkSkipChildren
// WalkContinue indicates that Walk can continue to walk.
WalkContinue
)
```
This commit the `iota`-based constants in the AST package to follow
this.
