7.1.0 - Expose all exposable types from TypeMap

Exposable types can now be enumerated as a complete list, e.g. for
endpoint discovery or API documentation generation.

New: EnumerableExposableTypeMapInterface with getExposableTypes(),
returning a list. The default ExposableTypeMap
implements it.

The interface is separate from ExposableTypeMapInterface, so
existing custom type maps keep working unchanged. Consumers that
need enumeration should check for the new interface instead of
assuming it on every ExposableTypeMapInterface.

See #31.