Add gitlab-ci.yml configuration file

This change adds a configuration file for the Gitlab CI/CD pipeline to
the repository. This file (directly or indirectly) controls the
environment in which to build, what exactly to build, and ultimately
would also be used to define what tests to run.
With the pipeline in place the change also adds a badge indicating the
status of the build on the project's front page.

The following other CI services have been evaluated or tested and found
to be insufficient for the needs at hand:
- Travis CI: Does not support configuration in a directory other than
             the repository root
- Cirrus CI: Has the same problem
- Semaphore: Does not support Rust
- Circle CI: Does not support Rust
- Google Cloud Build: Pretty much strictly a paid service

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