add CI workflows

[danieleades]

Adds CI workflows:

• pin dependencies by committing lockfile
• use dependabot for dependency bumping and lockfile maintenance
• check tests and formatting in CI
• bump MSRV to actual current MSRV and check it in CI

[danieleades]

you can see the results of the CI checks here - danieleades#2

[VirxEC]

Normally I probably wouldn't want/need something like this. However, since this crate is now very stable and not undergoing active development, I suppose this is appropriate.