stringer: avoid if's in the generated code

Suggestion by dsymonds: Save code not data.
Add an extra element to the index array and an if can be eliminated.

Old generated code:
const _Day_name = "MondayTuesdayWednesdayThursdayFridaySaturdaySunday"

var _Day_index = [...]uint8{6, 13, 22, 30, 36, 44, 50}

func (i Day) String() string {
	if i < 0 || i >= Day(len(_Day_index)) {
		return fmt.Sprintf("Day(%d)", i)
	}
	hi := _Day_index[i]
	lo := uint8(0)
	if i > 0 {
		lo = _Day_index[i-1]
	}
	return _Day_name[lo:hi]
}

New generated code:
const _Day_name = "MondayTuesdayWednesdayThursdayFridaySaturdaySunday"

var _Day_index = [...]uint8{0, 6, 13, 22, 30, 36, 44, 50}

func (i Day) String() string {
	if i < 0 || i+1 >= Day(len(_Day_index)) {
		return fmt.Sprintf("Day(%d)", i)
	}
	return _Day_name[_Day_index[i]:_Day_index[i+1]]
}

Change-Id: I6f46a4892d5813a12ec1ad01738c6a21c7e45172
Reviewed-on: https://go-review.googlesource.com/1990
Reviewed-by: David Symonds <dsymonds@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>