libffi 0.1.6

Rust bindings for libffi
# To do

  - Many more tests and examples.

  - Improve build process:
      - Why can’t we make bindgen a Cargo dependency?
      - Should we include a copy of libffi in the package?

  - ClosureOnce!
  - high::BuilderN?
  - Do we need to use `std::thread::catch_panic` somewhere?
  - Custom complex number types? But Rust doesn’t support complex numbers
    anyway...
  - CIF inspection?