- Added two new match functions, that still have to filled in.
- Division of identifiers yielded a rewrite possibility before, this is now
  fixed so that only numerics will be divided.

- Removed useless is_leaf check, since that is implicit in type checking.

- Now, integer- and float-leaves are never considered equal.

- 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.