Feature/Documentation: simpler FromPyObject for Python dict

[BlueGlassBlock]

The documentation of FromPyObject mentioned the usage of #[pyo3(item)], however I think it's quite feasible to have a #[pyo3(item_all)] on structs, which collects every field with get_item.

Apart from that, the pythonize project also provided the ability to build structs from Python dictionary, which worths mentioniing in the documentation as an alternative solution. However using #[pyo3(item_all)] should be easier for nested structures.

I'll work on the implementation and come up with a PR if #[pyo3(item_all)] is appropriate for PyO3 to have.