feat: specify msrv

[rinde]

With the stabilization of the minimum supported Rust version (MSRV) aware resolver it becomes more relevant to specify the MSRV of tinytime. I used cargo-msrv to determine the number. I think that the version number is relatively recent because we use the expect annotation which was only stabilized in version 1.81.0.

closes #6

[fabian-braun]

I think, if we specify a MSRV we should also have automated tests to check that the crate compiles with that version. WDYT?

[rinde]

@fabian-braun I think this is done automatically. If you specify a lower version, linting will fail on the specific functionality that doesn't work at that version. I think this is possible because the standard lib has every method annotated, e.g. #[stable(feature = "vec_resize_with", since = "1.33.0")]

[rinde]

@fabian-braun I think lint doesn't always pick it up so I also added a msrv-check

[fabian-braun]

Nice, I knew that other projects have a MSRV check in CI, so I was surprised that there's a lint for it. Maybe the check is also advisable when you depend on other third-party crates.