New API entry lint, minor incompatibility level

[epage]

Technically, semver says that these are minor changes.

Not all want to treat it as such and is an example of why people would want control over lints treatments, see #58

[obi1kenobi]

Blocked on #58, since my understanding is that Rust has not thus far issued strict guidance on minor vs patch, and if we build this before #58 then there's going to be guaranteed pushback.