Flake: add package, module

[s1341]

This PR adds a package and a nixosModule to the flake.nix.

For ease of testing, I also added a 'container' nixosConfiguration, which can be brought up by doing the following:

sudo nixos-container create asciinema --flake .#container
sudo nixos-container asciinema start

You should then be able to do:

export ASCIINEMA_API_URL=http://asciinema:4000
asciinema rec test.cast
asciinema upload test.cast

Note: for the package, it was necessary to set hashes on both the mixRelease and the rustPackage. Those will need to be updated as the code/dependencies change.