nix: various improvements

[pnmadelaine]

This PR improves the Nix code in various ways.

List of changes:

• centralize the Nix expressions in .nix/
• use z3 4.8.5 as defined in nixpkgs
• propagate the fstar-lib dependencies
• better formatting using nixfmt-rfc-style
• enable building on macOS and aarch64-linux (not tested)

I want to point out a regression in this PR: the size of the closure of F* basically doubles since propagating the dependencies brings back the OCaml compiler as a dependency. IMO this is a good trade, since compiling generated OCaml will now just work (tm). Currently, fstar-lib dependencies need to be manually imported, wich is not intuitive at all.

Also see https://github.com/NixOS/nixpkgs/pull/275924 which motivated me to do the propagation of fstar-lib dependencies.

cc @W95Psp

[W95Psp]

Hi! What about splitting the derivation, with a lib output, a bin one, and the default out output (containing lib and bin)? That'd address the issues without doubling the size of the closure if one don't want to