Add initial crypto functionality

Adds the functionality from the crypto spec:

1. Generate random Tracing Keys.
2. Generate Daily Tracing Keys.
3. Generate Rolling Proximity Identifiers.
4. Match a list of collected beacons (RPIs) with a list of downloaded
diagnosis keys (DTKs).

Includes unit tests, but these don't currently use the official test
vectors because I don't yet know how I can get hold of them.