Iris as submodule; CI support

This adds Iris as a submodule, matching the structure in https://gitlab.mpi-sws.org/FP/LambdaRust-coq/. Furthermore, it sets up the CI so after pushing, the repo is built against the given Iris version (with a cache to avoid needless rebuilding of Iris).

@zhangz if you have any unpushed changes, please push them. This branch moves all files to a subfolder theories/, so it will conflict with any other change.