New FEC framework, with unit tests.

For review. This works, but it's not to be merged just yet because nothing uses it yet. The new post-db4o client layer will use it heavily.
Yup, `com.onionnetworks.fec.FECMath` precomputes some tables. Still not sure if it's worth the trouble, because FEC en/decode is a relative seldom operation and quite a few instances would have to be cached to have a reasonable chance for a cache hit.