Macro ffi_support::implement_into_ffi_by_delegation

macro_rules! implement_into_ffi_by_delegation {
    ($SrcTy:ty, $MidTy:ty) => { ... };
}

Implement IntoFfi for a type by converting through another type.

The argument $MidTy argument must implement From<$SrcTy> and [InfoFfi].

This is provided (even though it's trivial) because it is always safe (well, so long as $MidTy's [IntoFfi] implementation is correct), but would otherwise require use of unsafe to implement.