add shell.nix file for Nix users

[dermetfan]

NixOS users may need this file to build Cursive if they don't want to install ncurses in their user profile.

[gyscos]

Thank you for your contribution!

I don't know much about NixOS; when would this file be required?
Would this help if a NixOS user tries to compile a rust application that depends on cursive?

[dermetfan]

when would this file be required?

Every time they try to build Cursive. NixOS users enter a shell environment with the dependencies declared in shell.nix present so they get reproducible builds.

In theory those dependencies could also be installed system-wide, but I just tried that and it fails for some reason. I guess nix-shell environments keep references to build dependencies that are removed when the dependencies are installed as applications for the user.

Would this help if a NixOS user tries to compile a rust application that depends on cursive?

In this case Cursive is fetched as a compiled library so shell.nix is not included. They would need to write their own shell.nix for that project. However, if we have a shell.nix here they can just copy our dependencies into theirs.

[gyscos]

If nix users use cargo, I think it downloads dependencies and compiles them locally?

Anyway, I can merge that if there's cases where it helps.

[dermetfan]

Even then, Cursive's shell.nix would not be downloaded into their project's root dir, so they'd need to type its full path somewhere in target/build/cursive-* every time. And if they need additional dependencies they still have to create their own file.

Cool, thanks!