Add TODO macros for more convenient aborting

[aleasims]

Right now we have some part of code, which use UNREACHABLE macro to abort because this part was not yet implemented. Is would be nice to have a special macros for these case, because "unreachable" seems to have different semantics.

We can add TODO(msg) macros which will abort the process and print something like:

NOT YET IMPLEMENTED at /path/to/source
    feature X is not yet implemented

Also we can have another macros TODO_WITH_LINK(msg, link), which will also print the link to corresponding issue if one exists. It will increase verbosity even more:

NOT YET IMPLEMENTED at /path/to/source
    feature X is not yet implemented
    Tracking issue: https://github.com/NilFoundation/zkllvm-assigner/issues/...