Support libffi error status FFI_BAD_ARGTYPE

[tov]

Currently we recognize only FFI_BAD_ABI and FFI_BAD_TYPEDEF, which we translate to Error::Typedef and Error::Abi. As of now, FFI_BAD_ARGTYPE gets translated to Error::Abi (which maybe isn’t strictly incorrect?). But it would be better to add a third enumerator, Error::Argtype for representing FFI_BAD_ARGTYPE. This isn’t difficult, but it will require a major version bump.