YubiKey NEOs are legacy YubiKey devices, most of which contain
unpatchable security vulnerabilities.
They have smaller buffer sizes than YK4 and YK5, which necessitates a
whole bunch of conditional gating and buffer size calculations.
Getting rid of them simplifies this logic and allows us to assume
consistent buffer sizes everywhere.
We never tested on NEOs anyway, and looking at the deleted code it seems
it may have been miscalculating the NEO's buffer size!
If someone *really* wants to support NEOs, it shouldn't be that hard to
restore, but the codebase is definitely cleaner without it.
Adds an extremely basic initial test to ensure that we are able to
connect to a YubiKey.
The test is marked `#[ignore]` in the hope that we can eventually start
adding tests which run in CI, e.g. against a mock card.
This also includes a fix for calculating the APDU size, since the ones
we were sending originally were overly long.
Tony Arcieri