Setup Nix flakes for integration into HBB performance workflow

[ehildenb]

We have the ability to run integration tests here: #94

We want to make this as easy as possible for HBB team, which requires having flake.nix for it. This requires:

• flake.nix for wasm-semantics Add Nix flake build wasm-semantics#603
• flake.nix for mx-semantics
• flake.nix for mx-backend

[ehildenb]

@goodlyrottenapple and @Baltoli can probably help with Nixifying this stack.

[Baltoli]

We should write the process of doing this down so that it's easier next time we set up a nixified project.

[goodlyrottenapple]

I have started looking into this, but we should probably clarify what exactly the goal is. E.g. if we just need to run kwasm or the mx-backend tests, but don't want/need to distribute these projects via kup, then we can just use nix to build a shell with any dependencies like poetry,make, z3, etc., necessary to run some version of make tests. This is what we currently do for our kevm/kontrol performance scripts.

If we also want to make these projects available via kup, then it's obviously more involved and I will need to better understand how people expect to interact with these projects

[ehildenb]

We want kup install kasmer to install the entire toolchain eventually, so it can be used for MultiverseX verification engagements.