Add marshalling to Private (TPM2B_PRIVATE)

[brandsimon]

Good day,

I need this to load an object created with tpm2_create.

[Superhepper]

Thanks for the PR. It would also be great if you could add some tests for this as well.

[brandsimon]

@Superhepper good catch, done

[Superhepper]

Sorry for not noticing this the first time but it is better to implement the marshall_offset and unmarshall_offset because the default implementation of marshall calls marshall_offset with offset=0.

[brandsimon]

@Superhepper no worries, code looks cleaner with the _offset methods, I like it! :)