Transaction::from_json holds an input txid that is not 64 hex digits and writes the input as spending the zero txid

Summary

`Transaction::from_json`, for a document with no `hex` member, held each input's `txid` member as given. An input `txid` that is not 64 hex digits (`"zz"`, `""`, 63 or 65 digits, characters that are not hex digits) answered `Ok`: `source_txid` and `to_json` gave the text back, while `to_binary` wrote the input as spending the txid of 32 zero bytes and `id()` hashed those bytes. Documents naming different texts were one transaction, spending an outpoint none of them named.

An application that reads a peer's transaction document with `from_json` and relies both on the text of its inputs and on its bytes or `id()` is affected; one that reads only documents it produced is not.

0.8.6 refuses such a txid (`Invalid txid of input N: expected 64 hex digits`). Where upgrading is not possible, require each input's `txid` to be 64 ASCII hex digits before calling `from_json`.