Rust: Allow sending `SimpleCapParams` across threads

[oli-obk]

also

• adds a travis build script (which tests building the C-part, too, as the Rust part builds that and links against it)
• small cleanups using the correct type aliases

[oli-obk]

In order to enable travis, you'd need to activate this repository at travis-ci.com by logging in with github and finding the repo in your user-settings. You can see how this looks at my fork: https://github.com/oli-obk/escapi/runs/123030626