Switch to Rust 1.31 in CircleCI and enable all tests

[ebkalderon]

Changed

• Change cargo fmt to cargo fmt --all in CircleCI.
• Switch minimum required Rust version in CircleCI to 1.31.0.
• Switch to custom Rust CircleCI image with RenderDoc (ebkalderon/renderdoc-rs-circleci-dockerfile).
• Enable all unit tests and doc tests in CircleCI.

This pull request should allow all tests (including doc tests) to be compiled and run automatically before merging, preventing failure cases in CI like in #44 from happening again.