Allow verifying against a library version X, but running against a newer version X+N

[keyboardDrummer]

Allow verifying against a library version X, but running against a newer version X+N if its contract changed in a backwards compatible way, such as new methods or postconditions being added.

This feature becomes important when there is a Dafny library ecosystem. Without it, if a low level library, such as the Dafny runtime, updates, then all directly or indirectly consuming libraries need to update as well. With this change, a low level library can release a backwards compatible new versions without requiring other libraries to release an update as well.