Add lint for fallible impls of From

[HMPerson1]

Fixes #691.

Some notes:

• I'm note sure about what to put for the error messages or documentation, so I would love any suggestions about that. Also maybe it should include a help string?
• It reports the calls to begin_panic instead of the macro panic!() so it says note: this error originates in a macro outside of the current crate, which might be confusing.
• It only detects calls to unwrap made with method-calling syntax; it won't detect UFCS syntax (i.e. Option::unwrap(foo)). AFAICT this applies to everything in methods.rs too.

[clarfonthey]

I disagree with this lint because FromStr is being deprecated in favour of From<&str>, TryFrom<&str>, and co., and I'd rather not move backwards.

[HMPerson1]

Well once TryFrom stabilized, it would be fairly easy to adapt this lint to say to use TryFrom instead of FromStr.

[clarfonthey]

IMHO From<String> is a very valid trait to implement under that, and the idea is that From here indicates an infalliable conversion. The point of moving away from FromStr is because it doesn't have that kind of guarantee.

[HMPerson1]

Which is why this lint only fires when we see a panic!() or unwrap in the implementation of from, suggesting that the conversion is fallible. See the discussion in #691.

[oli-obk]

We can make the lint allow-by-default and have it suggest TryFrom. That would not start suggesting a soon-to-be-deprecated trait, but also not suggest stuff to stable users that they can't use.

[oli-obk]

I'd be willing to merge this as allow-by-default as it is, but to make it warn-by-default I'd like to run it on a bunch of big crates and see how it fares. Do you want to do this before merging, or do you want to make it allow?

[oli-obk]

Nevermind I'm stupid. It's already allow out of other reasons...