Session 4 (Extending Numba) should mention `__hash__` and `__eq__` methods of types

Failing to implement these results in weird behaviour for parameterised types:

- `__hash__` is required for correct interning.
- `__eq__` is required to determine if casts are required.

Open Session 4 (Extending Numba) in the tutorial and locate the discussion of parameterised types. Add the stated __hash__ and __eq__ guidance, preserving the explanations of interning and cast determination. Done when the session explicitly warns about both methods.

Written by the indexing model from the issue text.