Add a `corebuild` script to Core which wraps ocamlbuild with all
the recommended options for building Core core from the command-line.

Signed-off-by: Yaron Minsky <yminsky@janestreet.com>
Signed-off-by: Anil Madhavapeddy <anil@recoil.org>
Merge branch 'master' into add-corebuild
core: disable errors-for-warnings by default
Merge remote branch 'mirage/add-corebuild' into core-109.34.00
add corebuild to core-109.34.00