Last layer_size entry in a1lx is redundant

[tdaede]

The last layer size can simply be inferred to be the entire Item size.

Not only does this shrink layer_size to only 3 entries, but by not coding the largest layer size it makes it much more likely to be coded as a u16, saving up to 10 bytes in the best case.

[leo-barnes]

I'll hold off on #131 until we've decided since it's using the a1lx box.