These module definitions were previously in abstract_value, abstract_location
and abstract_domain. They are now grouped in this new file for the three
abstractions.
These module types should only be useful for the engine, and need not to be
visible in the implementation of the various abstractions.

New module type Leaf in abstract_value, abstract_location and abstract_domain
with the key identifying the module. The internal structure is no longer needed
in these abstractions, and is instead built by the engine from this key.

Keys are no longer exported outside the modules.
For abstract domains, the key is automatically created by the functor
Domain_builder.Complete from the domain name.

Many changes throughout the analyzer:
- in the engine, Abstract_value.{Internal|External} becomes
  Abstract.Value.{Internal|External}.
- in the abstractions, Abstract_value.Internal becomes Abstract_value.Leaf.