Init bxCAN receive filters to accept all messages.
Group bxCAN receive registers at the Haskell level.

This allows metaprogramming that abstracts over which receive mailbox
to use.
Provide API for configuring filters.

Because I'm tired of re-reading the reference manual every time I want
to try a different filter configuration.
Implement CAN message receive.

However, it doesn't actually work, and I can't tell why not yet.
bsp-can-test: Toggle an LED on any received CAN message.

Initialize the filters to accept anything.