Merge branch 'ci' into 'master'

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.

See merge request !2
2 jobs for master in 2 minutes and 39 seconds
Status Name Job ID Coverage
  Iris
passed iris #1256
coq

00:00:11

 
  Iris Atomic
passed iris-atomic #1257
coq

00:02:28