Generate std::ffi::* instead of the std::os::raw::* types

[nyurik]

std::os::raw recommends to use core::ffi instead, but for std crates using core directly seem to trigger some warnings. The std::ffi::* offers the same types like c_char and c_int as core::ffi::*, plus it offers the newer types like CStr in there. Would it make sense to consistently use just the std::ffi::* for all C types in the generated code?

P.S. I am aware that this can be done with .ctypes_prefix("::std::ffi") param - but the question is if this should be done by default?

[pvdrz]

I think core::ffi is used if the use_core option is enabled and the rust target is high enough. Isn't that the case?