TODO:
----
- Switch to the New CLN version.
- to_interval.
- Merge into the Number_types package ?
- Add min/max wrappers depending on what is decided for CGAL.
- Should I provide operator/(cl_I, cl_I) ?  Not sure.
- Things that are supported by Gmpz, and should (?) be ported to cl_I:
  - exact_division  (exquo())
  - approximate_decimal_length
  - convert_to<IA>
- The test suite needs to be more exhaustive...
  How about a generic NT test-suite ?  How does GMP do ?