[ffigen] Updated parsing of typedef enclosed declarations (#83)

* added getCursorUSR_wrap method to wrapper, and bindings

* Updated code to use libclang's USR for identification

* Struct's typedef name will be used now (if it is only used in 1 typedef)

* parseXXX now returns seen binding instead of null

* added tests and removed obsolete code

* updated version, changelog, test_coverage.dart

* added PR ref to changelog

* minor changes