async-wrap: expose async-wrap as binding

Expose basic hooks for AsyncWrap via the async_wrap binding. Right now
only the PROVIDER types are exposed. This is a preliminary step before
more functionality is added.

PR-URL: https://github.com/joyent/node/pull/8110


Signed-off-by: Trevor Norris <trev.norris@gmail.com>
Reviewed-by: Fedor Indutny <fedor@indutny.com>
Reviewed-by: Alexis Campailla <alexis@janeasystems.com>
Reviewed-by: Julien Gilli <julien.gilli@joyent.com>