[feature] #2491: Enum support in FFi

[mversic]

Description of the Change

Add support for converting data-carrying enums to and from FFI types. FFI equivalents for enums are not opaque, rather they are structs generated with FFI representation. Take the following enum as an example:

#[derive(IntoFfi, TryFromReprC)]
pub enum Enumeration {
    A(u32),
    B(PublicKey),
    C,
}

for IntoFfi will generate:

impl IntoFfi for Enumeration {
    type Target = __iroha_ffi__ReprCEnumerationIntoFfiTarget;
    
    fn into_ffi(self) -> Self::Target {
        ///
    }
}

unsafe impl ReprC for __iroha_ffi__ReprCEnumerationIntoFfiTarget {}

#[repr(C)]
#[derive(Clone, Copy)]
pub struct __iroha_ffi__ReprCEnumerationIntoFfiTarget {
    tag: u8,
    payload: __iroha_ffi__ReprCEnumerationIntoFfiTargetPayload,
}

#[repr(C)]
union __iroha_ffi__ReprCEnumerationIntoFfiTargetPayload {
    A: <u32 as IntoFfi>::Target,
    B: <PublicKey as IntoFfi>::Target,
}

for TryFromReprC will generate:

impl<'itm> TryFromReprC<'itm> for Enumeration {
    type Source = __iroha_ffi__ReprCEnumerationTryFromReprCSource<'itm>;
    type Store = (<u32 as TryFromReprC<'itm>>::Source, <PublicKey as TryFromReprC<'itm>>::Source);

    unsafe fn try_from_repr_c(source: Self::Source, store: &'itm mut Self::Store) -> Result<Self> {
        ///
    }
}

unsafe impl ReprC for __iroha_ffi__ReprCEnumerationIntoFfiSource {}

#[repr(C)]
#[derive(Clone, Copy)]
pub struct __iroha_ffi__ReprCEnumerationTryFromReprCSource<'itm> {
    tag: u8,
    payload: __iroha_ffi__ReprCEnumerationTryFromReprCSourcePayload<'itm>,
}

#[repr(C)]
#[derive(Clone, Copy)]
union __iroha_ffi__ReprCEnumerationTryFromReprCSourcePayload<'itm> {
    A: <u32 as TryFromReprC<'itm>>::Source,
    B: <PublicKey as TryFromReprC<'itm>>::Source,
}

Issue

Closes #2622

Benefits

Data-carrying enums can be used in FFI functions

Possible Drawbacks

Usage Examples or Tests [optional]

Alternate Designs [optional]

[mversic]

closed in favor of #2625 because it has the wrong target branch