Auto merge of #53013 - zackmdavis:infer_outlints, r=nikomatsakis

in which inferable outlives-requirements are linted

RFC 2093 (tracking issue #44493) lets us leave off these
commonsensically inferable `T: 'a` outlives requirements. (A separate
feature-gate was split off for the case of 'static lifetimes, for
which questions still remain.) Detecting these was requested as an
idioms-2018 lint.

Resolves #52042, an item under the fabulous metaïssue #52047.

It's plausible that this shouldn't land until after `infer_outlives_requirements` has been stabilized ([final comment period started](https://github.com/rust-lang/rust/issues/44493#issuecomment-408852946) 4 days ago), but I think there's also a strong case to not-wait in order to maximize the time that [Edition Preview 2](https://internals.rust-lang.org/t/rust-2018-release-schedule-and-extended-beta/8076) users have to kick at it. (It's allow by default, so there's no impact unless you explicitly turn it or the rust-2018-idioms group up to `warn` or higher.)

Questions—

 * Is `explicit-outlives-requirements` a good name? (I chose it as an [RFC 344](https://github.com/rust-lang/rfcs/blob/master/text/0344-conventions-galore.md#lints)-compliant "inversion" of the feature-gate name, `infer_outlives_requirements`, but I could imagine someone arguing that the word `struct` should be part of the name somewhere, for specificity.)

 * Are there any false-positives or false-negatives? @nikomatsakis [said that](https://github.com/rust-lang/rust/issues/52042#issuecomment-406409795) getting this right would be "fairly hard", which makes me nervous that I'm missing something. The UI test in the initial submission of this pull request just exercises the examples [given in the Edition Guide](https://rust-lang-nursery.github.io/edition-guide/2018/transitioning/ownership-and-lifetimes/struct-inference.html).

![infer_outlints](https://user-images.githubusercontent.com/1076988/43625740-6bf43dca-96a3-11e8-9dcf-793ac83d424d.png)

r? @alexcrichton

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