Add a `#[pyo3]` macro

It would be great to have a single #[pyo3] macro like the #[wasm-bindgen] macro in wasm-bindgen. This macro would have the following properties:

• It can be attached to functions, classes, impl $struct and impl $protocol for $struct. The macro can decide where it's attached to by using syn::parseinto a syn::Item, matching and delegating to the respective code gen function.
• It supports the parameters currently supported (mostly by reusing the existing code)
• Method types (static, method, etc.) are automatically detected. The constructor might require a #[pyo3(constructor)] annotation
• There is #[pyo3(module)] to replace #[pymodule]. It doesn't need to support #[pyfn] though since we can do freestanding functions now.
• Tests for all features. This doesn't need to be as elaborate as wasm-bindgen, but maybe we can come up with something where we feed syn item to functions in pyo3-derive-backend. (Integration tests are also sufficient)

The best was to get started is probably adding a function in pyo3cls/src/lib.rs that dispatches to the existing macros and then expanding upon that. Looking into capybara*s pyo3 wrapper and wasm bindgen's macro frontend should give some additional pointers.

Start in pyo3cls/src/lib.rs and inspect the existing attribute macros and their code-generation paths. Read how syn::Item is used, then compare the capybara wrapper and wasm-bindgen macro frontend mentioned in the issue. Done means one #[pyo3] dispatcher covers the requested item forms and parameters, with tests for the supported features.