Integration fixes - SHA256, Format v2, and inversion

[jcjones]

Matches up with the changes to rust-cascade in https://github.com/mozilla/rust-cascade/pull/22

Changes are:

• Don't put the hash algorithm in the top-level filter cascade header, as it's on each layer
• Move all python struct definitions and their enums to their own file, fileformats.py
• The hash iteration number has to be u32, not u8, because of how it was implemented in rust-cascade
• Fix a bug where the salt wasn't copied to the filters when they are constructed during initialize