ob-rust.el: Add `ob-rust', org-babel mode for rust

* contrib/lisp/ob-rust.el: Add ob-rust.el.  It has been adapted from
  ob-C.el and provides similar features:

  - `:var' assignments
  - `:uses' for module imports
  - `:externs' for crate imports
  - `:attributes' for setting attributes
  - Automatically wrapping code in `fn main() { [...] }' if necessary.
  - Only support for :results output
  - Not much in the way of error feedback