Bindings

[roberth]

Adds bindings, to let us call Nix from Rust.

Closes #29

[Radvendii]

• It was really bugging me so I figured out how to get () working in the check_call macro. Extra bonus is the formatter now kicks in. see #43
• check_call! really needs a clarifying doc comment. even something simple like

  check_call!() wraps a single C api call, making sure to check the context for errors.

• we discussed that requiring some references to be mutable doesn't really make sense because they can alias in the C code anyways. Is the situation for Contexts different?
• check_one_call_or_key_none() never got removed, but it's not being called anywhere