- Moved combine_numerics to new rules file and renamed it to add_numerics.
- Added (match_)divide_numerics that checks if numbers can be divided without
  losing precision.

- Addition, subtraction, multiplication of powers are covered, see
  src/rules/powers for more details.
- All cases have been unit tested.
- Match functions have been added to the rules list in comments, to be
  uncommented when verified.

All unit tests are converted to the new tree representation.

match_combine_polynomes will return (n0, n1, c0, c1, r0, e0), thus avoiding the
duplicated arguments. Adjusted the unit tests as well.