[Nix] A handful of missing features

Here's my to-do list for Nix (things I didn't include because I didn't know about them when I first wrote the tutorial):
- You can use interpolation inside a dotted expression, e.g. `nixpkgs.pkgs.haskell.packages.${compiler}.callPackage`
- Inherit, e.g. `inherit a b;` and `inherit (foo) a b;`
- You can use `inherit` in `let` binding context as well as in sets
- `@`-patterns similar to Haskell pattern matching. For example, in a function definition: `f = args @ {x, y, z}: ...;`
- `or` can be used to provide a default value when pulling a value out of a set, e.g. `x.y or "defaultY"`