Switch to integer encoding to little-endian

[wemeetagain]

ethereum/consensus-specs#139 got merged.
Integers should now be encoded in little-endian format.
This includes prefixed lengths and encoded integers.

[GregTheGreek]

Just saw this, would be happy to take a stab at this unless you do?

[Mikerah]

A little food for thought: Would it be valuable to have the option to encode integers in big endian? SSZ has some really nice properties that others might want to leverage in other projects that may or may not be related to Ethereum. They might require integers encoded in big endian. This could be a future feature and will not be a priority within the next few months.

[GregTheGreek]

@Mikerah Just saw this. We can easily just cherry-pick the old code and add support for it. Ill make an issue for the backburner