Code Reorganisation

- Move three functions from `bitcoin.py` to `util.py` as a temporary
  measure.

- Moved the rest of `bitcoin.py` to `transaction.py`.

`bitcoin` is a terrible name for a module (for obvious reasons), and
most of what was contained in it was related to transaction construction
and serialisation. This move, however, is incomplete, as some of the
code that is now in `transaction.py` should actually be elsewhere.

Updated test suite to deal with new module name.
More Code Reorganisation

Finished moving the appropriate functions out of `transaction.py`. Put
all address‐related functionality into `script.py`.

Minor changes to test suite to make this work.