Efficient storage of keys and levels

[wismill]

In #626 I added some bounds to avoid memory issue with big sparse arrays. We surely can do better:

• Use a hashmap?
• Use a continuous array, then do binary search at some index (e.g. soon after the X11’s 248 limit)?
• etc.

[bluetech]

The array is just so simple and convenient :) My inclination is to stick with it unless someone asks to use some really high keycode or level.

[wismill]

unless someone asks to use some really high keycode or level.

@bluetech it happened: see #849.