Core design and refactoring

[jaripekkala]

Should we depend on the bloc and flutter_bloc packages so heavily?

If we decide not to we could separate the form handling core from the flutter stuff.

Tests are on hold until refactor.

[jaripekkala]

WIP in dev branch. Also touches a lot for #1

[jaripekkala]

Experimenting with class form states instead of using Maps.

https://github.com/vantageoy/formini/blob/a0195cc8fcdf0cdd79423c8545309b59cfa43112/test/example/user_form.dart

https://github.com/vantageoy/formini/blob/a0195cc8fcdf0cdd79423c8545309b59cfa43112/test/example/user_form_state.dart

@tommiperttila you asked for better typings and here are they. Any feedback/questions?