[bdbarnett]

Fixes #40. #47 has merged, so this is based on main.

On CPython, dir() on a struct instance now lists the struct's fields and methods. Struct types resolve these by name in getattro, so object.__dir__ never saw them. Each struct type now has a __dir__ that adds them.

>>> sorted(n for n in dir(lv.grad_dsc_t()) if not n.startswith("_"))
['conical_init', 'dir', 'extend', 'horizontal_init', 'init_stops', 'linear_init', 'params', 'radial_init', 'radial_set_focal', 'state', 'stops', 'stops_count', 'vertical_init']

The MicroPython build already listed fields. On the current Unix build, dir(lv.grad_dsc_t()) shows stops, params and the gradient methods. The new test checks it there too.

The new smoke test test_struct_dir failed on CPython before this change with dir(grad_dsc_t()) is missing ['stops', 'dir', 'extend', 'params', 'linear_init']. It passes after, along with the rest of the smoke test. pytest (109) passes and regenerate_all.sh --check matches.