Lints related to function ABI changes

[staniewzki]

Original description:

• things like extern "C" fn: https://docs.rs/rustdoc-types/latest/rustdoc_types/enum.Abi.html
• new lints enabled by this:
  • [x] ABI kind changed, e.g. extern "C" fn to extern "system" fn (or, if extern is removed, to the default extern "Rust" fn)
  • [ ] ABI unwind ability removed, e.g. extern "C-unwind" fn to extern "C" fn
  • [ ] is moving from extern "C" fn to extern "C-unwind" fn a breaking change?
    • technically, yes, but it's like adding #[non_exhaustive] when it was first introduced -- it's fairly new so let's not penalize early adopters