Planned API breaking changes - umbrella issue

[wprzytula]

Not to overlook any planned breaking API change scheduled for before we release 1.0, I group them here.

Tasks

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1. #978 - cleaner and more flexible API, more type-safety, possible efficiency gains; mind #477 and #475;
2. #905 - multiple users got confused; mind #890;
3. #977 - so that the structure makes more sense; mind #618;
4. #637 - for smoother user experience, especially at first steps;
5. #595 - solves #786, enables solving #691;
6. #462 - large deserialization refactor providing new features as well: #271; mind #432;
7. #519 - rethink errors; mind that in #462; mind #702;
8. #662 - consider this and decide if we change it before 1.0;
9. #579 - consider this and decide if we change it before 1.0;
10. #385 - consider this and decide if we change it before 1.0;
11. #364 - consider this and decide if we change it before 1.0;
12. #771 - revisit the whole public API before release 1.0.