Merklize the TokenType table, and store off-device

[keepkeyjon]

The table of ETH tokens is growing quite dramatically. As a space-saving measure, we need to merklize it so we can store it off-device.

To do this, let's add a protobuf version of TokenType, and construct the table as a merkle tree. We massive space savings, since we'll only need to store a single hash in the firmware. Clients will then have to construct a TokenTypeAck merkle proof in response to a TokenTypeRequest from the device. As an added bonus, we could also support ShapeShift-signed, or even self-signed table entries, allowing for the addition of unlimited tokens at the user's leisure.

One big drawback from this strategy is that clients will need different merkle trees each time we add/remove/change something in the table.

[keepkeyjon]

FWIW, the table in question takes up 47k.