WIP: type annotations (#2877)

* typing for core.common and core.utils

* Cleanup of core.common core.utils

* Remove ambiguous overload

See: https://mypy.readthedocs.io/en/latest/more_types.html#type-checking-the-variants

* dims are generic Hashable; support for Python 3.5.0~3.5.2

* fixes

* fixes

* More polish for common and utils

* More polish for common and utils

* More polish for common and utils

* code review

Author: crusaderky