This PR adds a verification of `Array.qsort` properties, trying to use
`grind` and `fun_induction` where possible.
Currently this is in the `tests/` folder, but once `grind` is ready for
production use we will move it out into the library.
Note that the current `qsort` algorithm has quadratic behaviour on
constant lists, and needs to be adjusted. We'll only move the
verification out into the library once this has been fixed (and the
proofs adapted). These verification theorems may be commented out in the
meantime if it's urgent to fix `qsort`.
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Co-authored-by: Kyle Miller <kmill31415@gmail.com>
Co-authored-by: Joachim Breitner <mail@joachim-breitner.de>
