Use Result as a replacement for ControlFlow in Visitor

[LeSeulArtichaut]

This PR (almost) resolves one of the differences between rustc's TypeVisitor and chalk's Visitor by adopting rustc's design with std::ops::ControlFlow.

Supersedes #645 with another approach suggested by @jackh726: instead of defining an enum ControlFlow, define a type alias type ControlFlow = Result, which allows to use ? instead of a custom macro.

[nikomatsakis]

Seems good to me

[bors]

☔ The latest upstream changes (presumably #648) made this pull request unmergeable. Please resolve the merge conflicts.

Note that reviewers usually do not review pull requests until merge conflicts are resolved! Once you resolve the conflicts, you should change the labels applied by bors to indicate that your PR is ready for review. Post this as a comment to change the labels:

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author

[jackh726]

Gonna close this in favor of #645. Thanks anyways @LeSeulArtichaut for writing an alternative implementation here. Overall, it's a neat idea, but I think the simple approach is probably better.