Combinator interface for configuration

The Mirage 1.1.0 release features a new combinator interface to make it easier to map device drivers in config.ml into concrete applications. This breaks backwards compatibility with Mirage 1.0 configuration files, but the added benefit is significant, so we felt it was worthwhile.

The types directory also now contains the V1 and V1_LWT module types used throughout the Mirage libraries.