Add 'lib/AquaticPrime' submodule

Will be using the `CoreFoundation` library code to handle licensing for
DomeKey.

Set this up as a sparse checkout to only checkout the
`Source/CoreFoundation/` path from the repository. Doesn't seem like
this is going to carry over for other people who `git submodule init`
though. Not sure how this works.

Found these resources on how to set up a sparse checkout:
- https://briancoyner.github.io/2013/06/05/git-sparse-checkout.html
- https://stackoverflow.com/questions/6238590/set-git-submodule-to-shallow-clone-sparse-checkout

Followed a Stack Overflow post by 'max630'
(https://stackoverflow.com/users/2303202/max630) to set up the submodule
as a sparse checkout:

    git init
    # I did not find a way to add submodule in 1 step without checking out
    git clone --depth=1 --no-checkout ../sub sub
    git submodule add ../sub sub
    git submodule absorbgitdirs
    # note there is no "submodule.sub.sparsecheckout" key
    git -C sub config core.sparseCheckout true
    # note quoted wildcards to avoid their expansion by shell
    echo 'foo/*' >>.git/modules/sub/info/sparse-checkout
    git submodule update --force --checkout sub

(https://stackoverflow.com/questions/45688121/how-to-do-submodule-sparse-checkout-with-git/45689692#45689692)

Ran the following commands to set up the sparse checkout submodule based
on the instructions in the post:

    $ git clone --no-checkout https://github.com/bdrister/AquaticPrime.git lib/AquaticPrime
    Cloning into 'lib/AquaticPrime'...
    remote: Enumerating objects: 797, done.
    remote: Total 797 (delta 0), reused 0 (delta 0), pack-reused 797
    Receiving objects: 100% (797/797), 829.07 KiB | 599.00 KiB/s, done.
    Resolving deltas: 100% (343/343), done.
    $ git submodule add https://github.com/bdrister/AquaticPrime.git lib/AquaticPrime
    Adding existing repo at 'lib/AquaticPrime' to the index
    $ git submodule absorbgitdirs
    Migrating git directory of 'lib/AquaticPrime' from
    '.../DomeKey/lib/AquaticPrime/.git' to
    '.../DomeKey/.git/modules/lib/AquaticPrime'
    $ git -C lib/AquaticPrime config core.sparsecheckout true
    $ echo 'Source/CoreFoundation/' > .git/modules/lib/AquaticPrime/info/sparse-checkout
    $ git submodule update --force --checkout lib/AquaticPrime

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