Based off feedback from lilyp and rekado_ on #guix, I've made some improvements:

- Remove the recursive checkout

- Extract mimalloc as a (nonpublic) package

- Fix commit hash value

- Enabled tests

I've made an amend commit, so the new patch replaces the original one above.

Note that the previous commit hash of solvespace of was off. The correct hash for solvespace 3.1 is 70bde63cb32a7f049fa56cbdf924e2695fcb2916, as shown in the new patch.

Thanks for all help this far, and hoping to see SolveSpace included in GNU Guix.

- Kris