Make CFRunLoopMode a safe wrapper with a lifetime

[kevinmehall]

Fixes #648: the safe methods that take CFRunLoopMode are currently unsound because it is a raw pointer that is not necessarily valid.

As a bonus, this allows use of kCFRunLoopCommonModes and kCFRunLoopDefaultMode constants without unsafe.

I considered just making mode a &CFString, but it seems that the implementation compares the pointer values, so this way ensures that the constants are passed as the pointers it expects. I don't understand the purpose or usage of mode very well, so correct me if this is not ideal.

[waywardmonkeys]

Hello! Sorry for the long delay.

Would you be able to rebase this forward and fix the merge conflicts? If so, we'd like to review it and see about landing it.

[kevinmehall]

Rebased

[kevinmehall]

Clippy doesn't like methods called default:

error: method `default` can be confused for the standard trait method `std::default::Default::default`
  --> core-foundation/src/runloop.rs:37:5
   |
37 | /     pub fn default() -> CFRunLoopMode<'static> {
38 | |         unsafe { CFRunLoopMode(kCFRunLoopDefaultMode, PhantomData) }
39 | |     }
   | |_____^
   |
   = help: consider implementing the trait `std::default::Default` or choosing a less ambiguous method name
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#should_implement_trait
   = note: `-D clippy::should-implement-trait` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::should_implement_trait)]`

so I renamed the constructors to CFRunLoopMode::default_mode() and CFRunLoopMode::common_modes(). But if you think that's too redundant, we could also ignore the clippy lint.

[waywardmonkeys]

I think the naming, while redundant, is fine.

[kevinmehall]

Pushed again to fix rustfmt on a line that default_mode pushed over the length limit.

(FYI, there's a GitHub Actions setting for "Require approval for first-time contributors who are new to GitHub" that is less annoying than the default "Require approval for first-time contributors ")