Implement dict views objects

[OutSquareCapital]

Currently we have the ABCs views (keys, values, items), and we use them to opaquely wrap PyDict.

It works! but, this is:

• suboptimal => we use PyObject functions instead of highly optimized PyDict ones)
• error prone => due to the fact that ABCs views and dict views do NOT have the same behavior).

However the C API surrounding views is a bit clunky and currently VERY limited in pyo3, so we should first of all choose if we:

A) Implement "from scratch" the views in pyclass, directly wrapping a PyDict.
B) Wrap the corresponding views objects.

B is vastly preferable due to better consistency with the rest of the codebase and much easier maintenance, but due to the aforementionned clunky C API this is not a straightforward a decision than other collections.

Moreover, the "1 parent class" only force us to first of all refactorize the rust codebase as a whole to consistently use py_abc proc macro to enforce traits and reduce code duplication, to avoid painfully reimplementing ALL the methods by hand again.