More Unpack API Calls; `BalanceError`s

Now, the `unpack` API method is defined automatically for all message
modules for which there exists an `unpack()` function.

Also, differentiate between balance errors and validation errors for new
message parsing code.

Solves #352.
Optimize `get_tx_info2()`.

Exit early from `get_tx_info2()` if the transaction in question isn’t a
burn and it contains no data; avoid looking for sources, which is the
expensive part.

Solves #542.
Merged `address.py` into `script.py`

A library named 'address' is a bad idea.
allow manually providing pubkeys for multi‐sig (fixes #413)