Use generators for walking in:

- find_index
- find_last_index
- find
- find_last
- for_each
- for_each_right
- for_in
- for_in_right
- transform
- is_monotone
Add has_path() and get_path().
Add to_string().
Refactor imports:

- Embrace "circular" imports by using pydash main API within functions
instead of importing from source files. Started to get too messy trying
to avoid interdependency imports.
- Move non-API helper functions to utils.py.
- Moved has() back to objects.py.
Add "op" as valid name.
Add conjoin() and disjoin().
Add versionadded metadata to docstrings.
Add juxtapose().