Add a python-defined layer over the bindings.

[aborgna-q]

Move the bindings to a tket2._tket2 submodule and re-define all structures from python, backed by their binded counterparts.

See Quantomatic/quizx for an example.

This gives us more flexibility on the python interface, where we can define methods that better reflect the python conventions. Additionally, typing information is simpler and less error-prone; we only need the hand-written .pyi for the reduced binded methods and missing values will not affect the external API.