support ffi_export on freestanding functions

[mversic]

currently ffi_export supports only inherent impl blocks. Expanding the support to include freestanding functions should be very easy.

This function:

#[ffi_export]
fn some_fn(arg1: String, arg: &[u32]) {
    // ... do something ...
}

should produce, once macro is expanded, something like the following extern fn declaration:

extern fn __some_fn(arg1: <String as TryFromReprC>::Source, arg2: <&[u32] as TryFromReprC>::Source) {
    // ... generated code ...
}

[ales-tsurko]

Should it support functions with return types either then ()? And if so, what kind of type should be returned? <T as TryFromReprC>::Source?

[mversic]

Should it support functions with return types either then ()? And if so, what kind of type should be returned? <T as TryFromReprC>::Source?

Yes, the type returned is <<T as IntoFfi>::Target as Output>::OutPtr. You can see this in the impl_visitor I believe