Add bindings detection to bin targets

[messense]

Closes #311

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[messense]

@davidhewitt I wonder should we support abi3 bin bindings, if so I think BridgeModel needs a refactoring to separate bin/lib targets from pyo3/rust-cpython bindings crate. Something like

enum BuildType {
    Bin(Option<BridgeModel>),
    Library(BridgeModel),
}

enum BridgeModel {
    Cffi,
    Bindings(String, usize),
    BindingsAbi3(u8, u8),
}

[davidhewitt]

Based on PyO3/pyo3#2328 (comment) I understood that it's not really possible to link to libpython3.so (i.e. abi3 distribution) without fixing issues in upstream CPython.

[messense]

The Windows test failure is odd, it works when execute pyo3-bin.exe directly from terminal, but subprocess.check_output returns exit code 3221225781.