Parse partial length headers correctly.

[J08nY]

This adds support for partial length headers, currently only parsing them, not emitting them. With this PR PGPy can decode a partial length packet of indefinite length, however when outputing said packet back to bytes, it will be a packet with the usual 1 / 2 / 5 octet length, which means that packets with contents larger than 4,294,968,295 (0xFFFFFFFF) octets will be output incorrectly.

However, currently PGPy will not even parse those packets, so I believe this is an improvement that covers a large part of the use-cases of PGP.

To get PGPy to output partial length packets for packets larger than the 5 octet length limit, significant changes to the __bytearray__ logic will be required, as then, the packet header will need to insert bytes into the packet data itself. The current inheritance model doesn't make it easy.

[Commod0re]

This looks exactly like what I had in mind for being able to accept partial body lengths. We can revisit producing them later 8)