Reimplement Memory as seq<u256>

[DavePearce]

The goal here is to streamline the process of writing to memory in the normal case. That is, when a word is written at an aligned address. This should also simplify the logic for memory expansion.

Furthermore, it makes sense to drop Expand() altogether and simply have the various Write methods expand elastically. The reason for this is that it just reduces the amount of work required.