refactor the code

[kandu]

• refactor the code to make the internal logic more consistent
• Zed_string.Invalid exception

[pmetzger]

Are there changed tests to go along with the change in the API?

[kandu]

I've made a pre-release on my personal repository and tested it on both linux and windows. All seems good. Talk is cheap, let's do something unwise again :)

[kandu]

And after uucd, uucp are integrated as dependencies #16, we can do a refactoring again to improve the internal logic to support more kind of writing systems, e.g. Hangul Jamo.