Implement runtime availability checking

[madsmtm]

Add macro for runtime availability checking, with static assertions against the deployment target that allows eliding the runtime checks when unnecessary.

Part of #266.

Usage:

if available!(macos = 15.0, ios = 18.0, tvos = 18.0, watchos = 11.0, visonos = 2.0) {
    // Use API added in Xcode 16.0
}

// Or

#[cfg(target_os = "macos")]
if available!(macos = 15.0) {
    // Use API added that's only relevant on macOS
}

// Or

if available!(macos = 10.7, ..) { // Note the `..`
    // Use API introduced in macOS 10.7, and take this branch on all other platforms too
}

Runtime lookup implementation tested on:

• macOS 14.7, M1 Macbook
  • aarch64-apple-darwin
  • aarch64-apple-ios-macabi
  • x86_64-apple-ios-macabi
  • aarch64-apple-ios-sim
• macOS 10.14.6, Intel Macbook
  • x86_64-apple-darwin
  • i686-apple-darwin
  • x86_64-apple-ios
• iOS 9.3.6, 1st generation iPad Mini
  • armv7-apple-ios

TODO:

• Implement runtime lookup.
• Optimize runtime lookup with sysctl.
• Decide on the naming and syntax of the macro. Alternatives that I can think of are:
  • os_version!(macos = 15.0, ..)
  • version!(macos = 15.0, ..)
  • os_available!(macos = 15.0, ..)
  • available!(macos(15.0), ..)
  • available!(macos 15.0, ..)
  • available!(macOS = 15.0, ..)
  • available!(macos = 15.0, *)
  • available!(macos = 15.0, _)
• Use macro in examples. No current examples where it makes sense.
• Decide if a compilation error when using available!(macos = 15.0) on e.g. iOS is desirable, or if we just want to return false here?
• Document macro.
• Optimize runtime lookup with atomics.

[madsmtm]

Noting my reasoning behind the name/syntax:

1. The name "available" more closely matches the semantics, esp. at usage sites. E.g. "if available macos 15" vs "if os version macos 15". os_version sounds more like it'd return the actual OSVersion struct.
2. macos = 15.0 looks more cfg-like, and matches general expectations of that. Compare with e.g. cfg!(target_feature = "crt-static"). macos 15.0, while terser, looks odd compared to the usual Rust parse.
3. I chose lowercase macos because it matches the value of cfg!(target_os = "macos").
4. For the optional trailing .. vs. _, I chose .. because it generally carries the meaning of an "all wildcard", see both the the docs on destructuring and RFC 3681.
   *, as Swift uses, is overloaded in Rust to mean many more things, so I think that'd be confusing.

[madsmtm]

So, I tested using the available! macro a bit, both in winit and in wgpu, and found that making available!(macos = 10.13) a compilation error on iOS doesn't really make sense, there were many times where I wanted a piece of code to only run on a specific macOS version, and where #[cfg(target_os = "macos")] felt like overkill.

So instead, available!() without .. should return false for unspecified platforms, and with .. it should return true for unspecified platforms. I believe this also matches what Swift does.

[madsmtm]

Thanks for the review before @BlackHoleFox, really useful to get some feedback - if you feel like giving it another review, then feel free to do so now, I think I'm done with everything now.

I will try to upstream this implementation to the standard library later (perhaps under std::os::darwin?), but it's useful to have in objc2, both to get real-world testing, and to have it available now (instead of having to wait for it to be stable in std).