add PIN support to KeyData

[chrisccoulson]

(Draft until #348 has landed and this PR has some unit tests).

This adds PIN support to KeyData which is distinct from the existing
passphrase support.

Passphrases are used both for authentication with the hardware element
(such as the TPM) and for additional encryption on the host CPU with a
passphrase derived key, and are intended to use a memory hard key
derivation. The intention here is that this configuration provides some
additional protection in the event of a TPM compromise (eg, say a TPM
manufacturer is coerced by a government agency to provide firmware that
bypasses authentications), where sensitive data is able to be extracted
without the usual authentication, because extracting the secret from the
TPM will not be sufficient to obtain all of the key material necessary
to unlock a device.

PINs (in the literal sense) have a fairly low entropy - an 8 digit PIN
only has an entropy of 26.5bits, so this additional encryption will
provide little protection in the event of a TPM compromise - if
sensitive data is obtained from the TPM, the 26.5bits of entropy won't
provide a significant barrier to deriving the remaining key material
necessary to unlock a device. We take advantage of this by implementing
distinct PIN support that is only used for authentication. With this in
mind, the memory hard key derivation does not provide a lot of benefit,
so PINs only support PBKDF2, and it can be configured to run faster
than the key derivation for passphrases. In that sense, PIN support is
essentially just a faster and slightly weaker passphrase.

As the PIN is a PIN in the literal sense, it is encoded as a length
prefixed binary number before going through the key derivation.

This only implements the support to KeyData for now - unlocking support
will be added in another PR.