Add marshaling/unmarshaling methods for types that support it

[ionut-arm]

As mentioned here and here, it would be useful to have types on the interface of, for example, Public to marshal and unmarshal the underlying FFI type (e.g. TPM2B_PUBLIC). This would allow data to be moved between different machines that need to work with the same TPM-specific data structures.

These methods would defer to Tss2_MU_... functions exposed by the tss2-mu library (for which you can see the spec here), and so we can implement them for the types supported by this spec:

• TPMA_... types
• TPM_... base types
• TPMS_... types
• TPM2B_... types
• TPMU_... types

I expect these new methods to simply be extensions of the existing types, so adding them won't break the API.

[Superhepper]

In #293 I added traits for this that I also implemented for the Attest type in order to make things easier if one wishes to make a generic function for types that can be marshalled or unmarshalled.