Make clash-ffi a haskell library + FFI interface tests

[kleinreact]

This PR reorganizes clash-ffi to be used like a normal Haskell library as part of an external project.

To this end, clash-ffi now gets built as a standard Haskell library, which then is imported by the external project setting up the specific simulator actions. The external project is built as a foreign-library, which then can be loaded into the simulator. This approach introduces more flexibility in the sense that every specific simulation application results in its own VPI-loadable shared library. This is the way of usage, as it is intended by standardized interfaces, like VPI, VHPI or FLI.

A guideline for setting up a project using clash-ffi has been added to the README.md.

A small example project has been added to the example folder. It is a standalone project, since integrating it into the clash-ffi.cabal causes cabal to shorten the clash-ffi library link, making it more complicated to load the library inside the simulator.

The PR also introduces a test suite, which checks that data does not get corrupted during the transfer via the FFI. After introducing the tests some bugs could be identified, which have been fixed as well.

[martijnbastiaan]

A quick drive-by comment: cocotb builds one .so file (upon installation) and dynamically loads test benches / device under test drivers. We could replicate this behavior using hint. Still, the current proposal is an improvement over the status-quo. (And whether we want to go the hint route is up for debate.)

[martijnbastiaan]

Very nice. Could you add the example to CI? Just GitLab CI should be good. Let me know if you need help.

[martijnbastiaan]

Thanks! Feel free to (enable auto-)merge.

[DigitalBrains1]

I was just glancing through this and notice some natural language thingies.

[martijnbastiaan]

I really like the testsuite and the example you've added. LGTM!