Add initial flake for installing build dependencies for proof-systems

[Fizzixnerd]

This is the start of a very simple flake.

In scope for this version of the flake:

1. nix develop . will open a dev shell with everything you need to develop on various parts of proof-systems in scope (caveat about rust and ocaml toolchain version in next point).
2. You must still use rustup to manage your rust toolchains.
3. You must still use opam to manage your ocaml toolchains.

Out of scope for this version of the flake:

1. Distributed caching -- only if the others in this list come.
2. Actually building things using nix -- likely to come.

[dannywillems]

Can we have a continuous check in the CI that it works, and the code can be built, please?

[Fizzixnerd]

Can we have a continuous check in the CI that it works, and the code can be built, please?

Absolutely. I have tested

cargo build and that succeeds. What other checks would you like? My understanding is there is go and ocaml code somewhere in this repo.

[dannywillems]

Can we have a continuous check in the CI that it works, and the code can be built, please?

Absolutely. I have tested

cargo build and that succeeds. What other checks would you like? My understanding is there is go and ocaml code somewhere in this repo.

Would it be worth, even if the nix config is simple, to check in the CI that it works? By running, for instance, cargo build in the nix environment? IIRC, we do that in the Mina repository.

[dannywillems]

Approving as it is pretty simple. As a non-nix user, I simply tried the commands in the README.
Feel free to merge.

[Fizzixnerd]

Can we have a continuous check in the CI that it works, and the code can be built, please?

Absolutely. I have tested
cargo build and that succeeds. What other checks would you like? My understanding is there is go and ocaml code somewhere in this repo.

Would it be worth, even if the nix config is simple, to check in the CI that it works? By running, for instance, cargo build in the nix environment? IIRC, we do that in the Mina repository.

I will make an issue so I don't forget to.