Massively simplify pyconvert?

Don't take a type, so it's just `pyconvert(::Py)`. Symmetric with `Py(x)` for conversion in the other direction. Will be simpler so define rules and can just run up the MRO to apply them.

Could also have a `override` argument to possibly provide custom conversions (and same for `Py`).

Could additionally support `pyconvert(T, x)` as an alias for `convert(T, pyconvert(x))::T`. Or do something slightly snazzier which lets us skip over rules that cannot apply?