Full flake overhaul

[levydsa]

Improvements:

• Use rust-overlay, better maintained than fenix and allows for:
• Use rust-toolchain.toml as the source of truth for the current rust version, instead of tracking with stable. Preventing conflicting versions with non-nix users.
• Add flake checks, could be useful for CI in the future, together with crane and cachix.
• Add package, allow people to add limbo as a regular nix package. Now we can nix build .#, nix run .# and nix shell .# (this one adds limbo to the current PATH)
• Use new apple-sdk pattern, no need to declare each framework now.

[penberg]

@levydsa I am merging this and hoping that any issues that pop up, you just deal with. Including people complaining that the configuration does not match their preferences 😝. And this becomes a never-ending thing, we'll probably just drop the stuff from the tree.