Add corebuild script to Core

Add a `corebuild` script to Core which wraps ocamlbuild with all the recommended options for building Core core from the command-line.  We'd like to use this script in Real World OCaml to shorten the examples and enforce the recommended warnings.

Signed-off-by: Yaron Minsky <yminsky@janestreet.com>
Signed-off-by: Anil Madhavapeddy <anil@recoil.org>

/cc @diml -- could you review and approve this before merging? I've only tested on 4.01.0dev, but it should hopefully work on 4.00.1 as well.