[feature] #2491: Enum support in FFi

[mversic]

Description of the Change

• removed wasm feature flag for target_type. After some discussion with @Erigara this was decided to be preferable
• replaced IntoFfi and TryFromReprC with FfiType and FfiConvert traits. Previous separation couldn't be worked with mostly due to conflicting requirements on lifetimes on data carrying enums. Maybe GATs would have been of help here, not sure anymore. If needed FfiConvert can be split into TryFromFfi/IntoFfi
• removed lifetimes from all ReprC types (namely SliceRef/SliceMut). It was too noisy and difficult to deal with but I think it can (should?) be reintroduced now
• added non_robust_ref_mut feature flag. Might be useful to disallow mutation for non-robust types
• improved conversion system so that it can now convert any recursive type
• added implementation for data-carrying enums
• prevent double-free in function return via NonLocal

Type challenges

There is no specialization or HKTs in Rust, so it's very difficult to properly delineate type categories. But it can be emulated in the type system. This is what the Ir trait is for. Rust type is first wrapped in the correct Ir type after which static dispatch on IR marker types provides an optimal(zero-copy when possible) implementation of FFIType.

Issue

Closes #2491 #2629

Benefits

optimal conversion, enums support

Possible Drawbacks

I'm not sure if it's more complex than before of just a different perspective

Usage Examples or Tests [optional]

This is what a conversion for data-carrying enum would produce (it is not even required it is repr(C):

pub enum DataCarryingEnum {
    A(String);
    B
}
pub struct ReprCDataCarryingEnum {
    tag: u8,
    payload: DataCarryingEnumPayload,
}
union DataCarryingEnumPayload {
    A: ManuallyDrop<<String as FfiType>::ReprC>,
    B: ()
}

There was a suggestion to use a different enum representation. It can be modified. At the moment I found this easier to implement

Alternate Designs [optional]

I've tried some but all failed in expresivity

[Erigara]

lgtm