BUG-981: untangle interfaces

This patch untangles the interactions between the three classes that
constitute the binding generator implementation. This provides a
baseline for analyzing thread safety and ensuring its proper function.

TypeResolver is a new interface, which encapsulates the lookup services
provided by RuntimeGeneratedMappingServiceImpl to TransformerGenerator.

AbstractTransformerGenerator is a new abstract class, which defines the
interface of TransformerGenerator to the rest of the package, such that
Java classes do not have direct dependency on xtend code. Furthermore it
acts defines the interface which the xtend code expects of the Java
runtime.

Change-Id: I0a4fdc23aa25c69e45a3cea63aab0168ee2fe3ce
Signed-off-by: Robert Varga <rovarga@cisco.com>
(cherry picked from commit 607a44f23acde9c0f13f9375b2fcda157605cbbc)
Signed-off-by: Robert Varga <rovarga@cisco.com>