yang-model-api: introduce interfaces for specific integer types

Some users have a need to check for specific subclass of an IntegerTypeDefinition, e.g. int8, int16.

This is currently implemented by walking up the type tree to see what the root type is – which is inefficient, considering this could reasonably be achieved via an instanceof check.

Introduce Int8TypeDefinition and friends and implement them in yang-model-util. For Restricted and Derived types this requires appropriate subclasses and instanceof dispatch when we build the type.