Tests for New Pubkey‐Handling System; Misc.

- `script.is_fully_valid()`
- `script.is_pubkeyhash()`
- `script.make_pubkeyhash()`
- `script.extract_pubkeys()`

* Fixed typos found while making tests.
* Also, slightly improved docstring for `transaction.py`.
* Also, fixed minor regression.

Solves #570.