Add `Transaction`

[rustaceanrob]

I used what was in bdk-ffi with some minor changes:

• Renamed the error to EncodeError, as it is the same error for other consensus encodings. In the future we may want to deserialize more types using this error.
• Renamed serialize to consensus_serialize
• Explicitly clone when generating the Vec<TxIn> and Vec<TxOut>
• Made the tuple struct public, as other library developers may want to work with the transaction directly

I really like what Davidson did with his PR, as he allowed users to construct a Transaction from TxIn, TxOut, Version, LockTime. However, I decided to leave that out because it requires more types and discussion (should we support non-standard versions).

[rustaceanrob]

Updated with those suggestions

[rustaceanrob]

Updated once more. Seems like we have some conventions developing. We may want to document them somewhere