- Use correct type for "decimal"
- Ensure that we do not confuse a Python `bool` for an `int` (since
  `isinstance(True, int)` evaluates as True).
- Check on the bit-length of `int` values to check whether they should
  be considered "int" or "long" from Mongo's perspective.

This patch adds the missing test for $type in `test__mongomock.py`. Note
that the "array" type was purposefully ignored from `supported_types`,
as there is an issue that will be solved in a future patch:
`_type_op([1, 2, 3], t)` should return True for both `t = 'array'` and
`t = 'int'`.