Implement transaction serialization.

[hdevalence]

Following from #105 (comment):

2. The actual transaction encoding and decoding is not implemented, although there is a stub module in which it will live (transaction/serialization.rs). Implementing ZcashSerialize and ZcashDeserialize is a further chunk of work, but because all of the unrefined types are in place, it can be done independently of (1) by, e.g., parsing something as a [u8; 32] instead of some refined PublicKey type.

This issue is for implementing serialization for transactions. It probably makes sense to do this together with the next follow-on item from that PR:

3. The as-yet-unimplemented encoding and decoding is not tested. In addition to the transaction test vectors, we could define a proptest strategy to generate random transaction data to check that the encodings round-trip.

[hdevalence]

I can start trying to stub this out. To make this independent of #123, the first implementation for this PR should write the unrefined types as raw bytes and read them back to raw bytes. Then, as #123 is filled in later, we can change the serialization for each type that we refine.