[do not merge] Consider removing isinstance check from trampoline

[gatesn]

Pydust performs an isinstance check inside the Trampoline.unwrap function (called from py.as(T, pyobj)) since this provides a nice level of automatic type-checking when invoking functions from Python.

Of course.... runtime type checking has a cost. It's not always desirable, and in the case of the self argument in instance/class methods it's kind of unnecessary.

Currently Pydust types are all heap-allocated (vs static types) to allow us to compile for the limited ABI.

Our isinstance check delegates to the Python ffi.PyObject_IsInstance function, which requires us to give a PyType. We grab this PyType by performing an import of the extension module and grabbing the class object from that. I'm not sure if there's a better way for heap-allocated types? Maybe we insert some global state into our generated libraries that populates on first import? Open to any other ideas?

This seems to address the performance issues in #226 , but ideally I'd like to:

1. Offer an alternative to py.as when knowingly converting from PyObject to PyDust class struct. Provide fast conversion from PyObject to Pydust type struct #227
2. Avoid isinstance checks on self/cls parameters (probably using 1)