[cs] Only use __hx_cast when the types fail to unify (and are not Dynamic). Old behaviour can still have a fallback with -D cs_safe_casts. Fixed last failing test. Closes #2066
[cs] Do not allow failures on Travis \o/
[std] Rewrite Int64 as an abstract
[tests] Rewrite Int64 unit tests
[java] Implement abstract Int64
[cs] Implement abstract Int64