Use external "deriving"

[yallop]

This pull request removes the bundled deriving library, introducing a dependency on the version maintained by the Ocsigen project instead. The main drawback is the extra dependency; the main benefit is that a few thousand lines of code disappear.

The easiest way to install the external deriving is via OPAM. The process goes something like this:

1. Install OPAM (see e.g. these instructions)

2. Install a recent OCaml compiler:

   $ opam switch install 4.01.0
   $ eval `opam config env`

3. Install the deriving package:

   $ opam install deriving

4. Build Links:

   make