Initial version of mhl decoding seems to work.

We are just doing a minimal deserialization, which will store everything in memory
Added MHL deserialization as well as simple test.

Next is serialization, possibly with amendment of hashes for the entire contents.
Implemented minimal writing of mhl files.

A test revealed that these verify just fine with MHL.
Implemented signature of MHL seals.

MHL is fully compatible with files we have written like that, and we are
compatible with ones that MHL has written, using the signature only
if we can.