[FIX] Correctly resolved templated classes

- fix issue with import statement (by storing original / C++ classtype in the
  ResolvedClass)
- fix instance_map that now always contains an entry with the C++ class name
- fix _resolve_constructor which now correctly resolves the constructor
  even if multiple instances exist for one (templated) C++ class
- add Tests for 2 cases of templated classes
- fix tests for DeclResolver (e.g. test_singular now works)