Migrate as much as possible to LaTeX3

- [x] Preamble an option declarations
- [x] Create new variants of `size1x.clo` and maybe `fleqn`/`leqno.clo`
- [x] Command/environment definitions
- [x] Color themes
- [x] Everything else
- [x] Update all documentation #1
- [x] Regression test at least all the macros
- [ ] Test some or all options
- [x] Also regression test on some real-world documents
- [x] Use urdh/l3ctr2e