lint for `catch` used as identifier

[nikomatsakis]

In the new epoch, we would like to reserve the catch keyword for future use. Therefore, we should add a lint that catch is going to be a reserved keyword in the future.

[scottmcm]

Others that may or may not be part of this: https://internals.rust-lang.org/t/keywords-to-reserve-for-rust2018-epoch/6720

[nikomatsakis]

Yep, I plan to go through and vet that list, opening issues where appropriate.

[Centril]

Closing this and tracking a possible lint for try at #50412