CI: Run "cargo check" and "cargo fmt" on all commits.

This sets up two jobs on merge requests (not on all branches, only once we start making requests to merge them) that runs CI job to check that commits:

• pass through "cargo check" which is basically the compile-step but without the code-generation
• Pass through "cargo fmt" which is the official rust formatter

I have not tuned warnings etc. into aggressive state, as that's overkill until 1.0 release, and these jobs are allowed to fail. Even if I recommend that they shouldn't fail, it can always be a good reason for them not to succeed, like a new fmt / check rule happening and thus making "master" not clean, and might require more than one commit just to have neat commit history.

Thus, no hard rule that "it must always be clean" for rebase tests&checks, but still. I recommend it.