Add --strict flag to cargo msrv verify

[foresterre]

When --strict is enabled, the command will not only check if the given MSRV is compatible, but also whether it is the absolute minimal supported version. \

It can do this by also running an extra check for version x.(y-1) for a given MSRV x.y. x.y should succeed, while x.(y-1) should fail.

Consider the edge cases x.0