Lint for parameters that could be ?SIzed

[huonw]

Generic parameters being ?Sized is more flexible than them being the default of Sized, so we could detect the cases when ?Sized is likely to work to assist library authors in being as inclusive as possible.

(This likely would've detected #20744.)

[apasel422]

This would also have detected #29263.

[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!