subtype check should return all breaking changes

[chenyan-dfinity]

Currently, subtype check only return the first breaking change it finds.

[ByronBecker]

@chenyan-dfinity how much work would this take to get implemented?

From our perspective, we'd like to know everything that's expected to break before deploying. It's really helps with QAing deployments to make sure we didn't unintentionally break an interface or stable variable.