Add CI with latest dependencies

[GREsau]

Starting with #281, Cargo.lock is checked in to source control. This makes builds (especially for MSRV) much more stable, at the cost of no longer testing schemars compatibility with new dependency versions.

To improve test coverage, tests could be run both with the checked-in Cargo.lock (especially for MSRV), and without it so that we get the latest dependency versions (but probably not for MSRV!)