This PR makes two clarifications:

1. The SSZ-snappy length prefix (a varint) should be minimally encoded. Meaning that if, for example, a length can be encoded in 3 bytes it should be; rather than encoding the same value with more bytes, which is possible.
2. The specification wasn't exactly explicit that the length must be stored in a type that can represent all uint64 values. It was sort of implicit in my opinion, but best to make it explicit.

This was part of a report from the Ethereum Attackathon. Thanks Troy!