Adjust various typos / infelicities in documentation.

alloca already deallocates.

Can't assume /opt/arrayfire/lib exists.
Just add extra-lib-dirs w/ both lib64 and lib.

  - Adds stack.yaml
  - 3.8.2 bump fixes variance and product calculations in arrayfire-haskell tests
  - Tests on latest GHC versions

* Do not rely on == comparison for cbrt and sqrt tests

* Do not be too strict about the version constraint; upstream ArrayFire changes a lot between minor releases, so we check for that, but don't assert a specific patch version

* Switch to Nix flakes; make the tests pass with nix build & nix develop for the latest version of ArrayFire

* update flake.lock

Instead of allocating an array of pointers, joinMany was allocating
memory for just one pointer. This was making ArrayFire read out of
bounds and fail with various errors.

This commit fixes this issue by adding a helper withManyForeignPtr
function that acts like withForeignPtr (not unsafeWithForeignPtr!), but
for a list of ForeignPtrs.

The default implementation of (**) relied on log and incorrectly
handled some inputs. The fix is making an explicit implementation
using the pow function.

To test the changes, tests for functions from the Floating typeclass
are re-written using property tests. There are a few helper functions
to make writing the actual properties easy.

More tests can be converted to properties, but this is left for
another PR.

…ror. (#59)