Vsutil

- New functions:

* :func:`resolve_enum` added to public API (previously a private function used internally).
* :func:`get_lowest_value`, :func:`get_neutral_value`, and :func:`get_peak_value` for relevant min/median/max float values based on sample-type and bit depth.

- Changes to existing functions:

* Deprecated ``enforce_cache`` parameter of :func:`frame2clip`.
* remove deprecated APIv3 constants

- Add constant for `std.Expr` variables (56)
- Lots of documentation changes
- Improve internal enum usage

- Add `scale_value` function (44)
- Split single file into submodules (45)

Add `depth` function for depth conversion.