v3.0.0 simplifications

[timburks]

Noting this comment, I've started work in this branch to replace run() and action with the contents of run_with_result() and action_with_result, i.e. run() and Action will always return a Result. This is a breaking change, so it would be good to increase the major version if this change is made.

Along with the above change, I think it would be good to treat FlagError and ActionError consistently by renaming the current FlagError to FlagErrorKind and creating a new FlagError type that is similar to ActionError.

@ksk001100 Do you agree with the above changes for v3.0.0? Also, is there anything else (especially possibly-breaking changes) that you think should be included in v3.0.0?

[ksk001100]

@timburks
I think that policy is fine.