Idea: JSON marshalling

The [TCG TSS 2.0 JSON Data Types and Policy Language Specification](https://trustedcomputinggroup.org/resource/tcg-tss-json/) defines JSON serialization/deserialization rules for all the different TPM types.

I believe that these rules would make sense to integrate into this library. This is something I'd be willing to send patches for, though I would want to first get confirmation that this fits the scope of this library and doesn't belong somewhere else.