CI: add .gitlab-ci.yml + enable public testing.

Enables testing on Debian 9 (Stretch), with Python 3.5, 3.6, 3.7.

Builds lllc locally, statically, and shares the binary between all jobs on the same branch, using Gitlab's CI cache. Also shares the cache of downloaded PIP packages.

(Technically, could share the same cache absolutely everywhere, but might want to update the compiler's version later, and have the option of running different compiler versions across branches.

Also, this could make the PIP cache grow too fast.)