Implement String() and IsValid() via array, not map

[TeaDove]

I love go-enum, but maps for String() method could be slow and memory consuming. Bu we could use an index array instead, similar to the approach used by stringer tool:

Now:

var _ColorMap = map[Color]string{
	ColorRED:   _ColorName[0:3],
	ColorGREEN: _ColorName[3:8],
	ColorBLUE:  _ColorName[8:12],
}

// String implements the Stringer interface.
func (x Color) String() string {
	if str, ok := _ColorMap[x]; ok {
		return str
	}
	return fmt.Sprintf("Color(%d)", x)
}

Proposed

var _ColorIndex = [...]uint8{
	0,
	3,
	8,
	12,
}

// String implements the Stringer interface.
func (x Color) String() string {
	if x < 0 || int(x) >= len(_ColorIndex)-1 {
		return fmt.Sprintf("Color(%d)", x)
	}

	return _ColorName[
		_ColorIndex[x]:
		_ColorIndex[x+1],
	]
}

This avoids a map lookup on every String() call and should also reduce the amount of static memory required by the generated code.

Of course, this optimization only works when enum values form a contiguous integer range. Enums with explicitly assigned sparse values, string-backed enums, or other cases where array indexing is not applicable could keep using the existing implementation.

So the generator could choose between:

contiguous integer enum → index array;
sparse/non-contiguous enum → existing map-based implementation.

I'd be happy to submit a PR implementing this, including tests and benchmarks, if the maintainers are interested in this approach.