lint for matches with a single case

[thestinger]

Some of these are still around from when let patterns were totally broken. It'd also be nice from a pedagogical perspective.

[alexcrichton]

This may be a little difficult because match is still semantically a little different than let. I've found a number of match statements that can become let, but for example all the new formatting code uses a single-case match instead of a let for lifetime reasons.

[steveklabnik]

Traige: this lint still does not exist.

[steveklabnik]

Since new lints have a big impact on users of rustc, the policy is that they should go through the RFC process like other user-facing changes. As such, I'm going to give this one a close, but if anyone comes across this ticket and wants this lint, consider adding it to clippy and/or writing up an RFC. Thanks!