Extend CI to support multiple OSes

[AntoineRondelet]

For now, the CI checks only run on Linux (see: https://github.com/scipr-lab/libff/commit/864be23c08a957565f4a1b6b296887727aecebb2).

This contrasts, however, with the fact that the library can also be used on macOS and Windows. As such, and if we want to provide support for these platforms, we need to extend the CI to be able to run the tests and carry out builds on each of these platforms, as a way to keep a well tested and sound library.

See:

• Travis:
  • https://docs.travis-ci.com/user/multi-os/
  • https://docs.travis-ci.com/user/reference/windows/
  • https://docs.travis-ci.com/user/reference/osx/
• Github Actions:
  • https://docs.github.com/en/actions/reference/virtual-environments-for-github-hosted-runners