updates to Common Lisp libraries

In the past month I updated many of my Common Lisp libraries. Here is a brief summary of the changes:

1. cl-num-utils has been growing by accretion for a long time, which did not lead to the cleanest code. I decided to reorganize the library and split off functionally independent parts which may be useful on their own:
   • functions that operate on Common Lisp arrays have been moved to array-operations,
   • functions that handle array slices are now in cl-slice,
   • data frames are in cl-data-frame (which is still experimental, but works fine).

   The first two are meant to have very few dependencies, and in particular do not depend on cl-num-utils. I hope that cl-slice will serve as a basis for a Common Lisp API for slicing arrays and array-like objects.

   Parts of cl-num-utils have been disabled for now because I didn't have time to update their unit tests and I don't like to release untested code. If you need any of those, please open an issue on Github.

2. lla had to be rewritten a bit since it depends on cl-num-utils. I moved special matrices from the former to the latter since they do not require foreign functions per se, and moved array stacking code to array-operations.
3. cl-random also depends on cl-num-utils and had to be rewritten a bit. Again, not all functionality is enabled yet, feel free to open an issue if you need something urgently. I will re-enable everything in due time, but I would like to think more about testing multivariate distributions.
4. Minor changes were made to cl-rmath and let-plus.