rust: Use Cargo to build the Rust portion

Pros: Ability to use Rust libraries from crates.io
Problems: Need to lock library versions and document their reproducible building, either via Cargo.lock or vendoring.

.gitlab-ci.yml

@@ -9,6 +9,10 @@ stages:
     - librem5
 
 before_script:
+  - apt-get -y update
+  - apt-get -y install wget ca-certificates gnupg
+  - echo "deb http://ci.puri.sm/ scratch librem5" > /etc/apt/sources.list.d/ci.list
+  - wget -O- https://ci.puri.sm/ci-repo.key | apt-key add -
   - apt-get -y update
   - apt-get -y build-dep .