Gitlab CI should show coverage information, unit test pass/failure, linting status, etc.

[zv]

Gitlab pipeline output showing this branch's run

• Runs all clippy lint checks
• Warns if code does not conform to rust fmt / Polyswarm shop-style
• Instruments binaries produced by cargo with cargo-cov and automatically extracts coverage information, reporting a percentage in a CI badge included in README.md
• Reports unit test pass/failure through another CI badge in README.md

When merging, ensure the CI badges have their branch-status-URIs updated from feature/coverage-tests to master

Resolves #18 Resolves #13

Caching

A CI pipeline run could be sped up considerably by caching intermediate crate build output. I tried introducing the following YAML anchor to rust-related stages but didn't appear to be successfully caching intermediate results.

At this point, I don't believe it is a profitable use of anyone's time to spend cycles on the issue, however it may be worth coming back to in the future.

.rust-defaults: &rust-defaults
  variables:
    CARGO_HOME: $CI_PROJECT_DIR/cargo
  before_script:
    # we only want to build crates for one rustc & we need nightly for instrumentation
    - rustup update nightly
    - rustup default nightly
    - rustc --version
    - cargo --version
  cache:
    paths:
      - target/
      - $CARGO_HOME