shower thought: remove right/left padding when it makes sense

[gballet]

When a value is 0, 32 bytes worth of 0s are stored. This is very wasteful and could be improved by adding a special encoding to strip them out if it's worth it.

It needs to be assessed at serialization time if using e.g. rlp or rle saves space by counting zeros while serializing the leaves, and then choosing the methods: if there are more zeros than leaves, it's worth using rle.