- 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.