Creating new attributes at runtime lead to unclear typing and mypy errors.

As advertised by the type hint.

The changes in this commit don't affect runtime code.

- Added more type annotations
- Cleaned up and remove comments and docstrings
- Replaced `std_graph.features` with a keyword argument
- Made `DUPLICATE` an `object` sentinel
- etc.

- Move test description comments to docstrings
- Move optimization tests to `test_opt`
- Move `aesara.scan.views` tests to a new `test_views`
- Move example-based tests (i.e. mostly non-functionality-focused tests) to a
  distinct test suite/class
- Make rewrite tests specify the presence of the relevant rewrites (assuming
  that those tests even rely on their namesake rewrites, of course)
- Miscellaneous test updates