non_camel_case_types lint check should ignore enum variants

Given that enum variants are not types, it doesn't make sense to have a lint named non_camel_case_types be checking the naming of those.

Add another lint named non_camel_case_enum_variants to check those.

[steveklabnik]

Triage: no change.

I think it's fine to use 'type' in this context, but maybe it could be renamed to something else.

[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!