Incorporate CoreFoundation ideas from other crates

[madsmtm]

Go through the implementations in the following projects, and grab relevant ideas that objc2-core-foundation could use:

• core-foundation
• fruity::core_foundation
• core-foundationr
• cidre::cf

A few things I've identified already:

• Mapping of CFBase integer types: 5bc094b.
• CFType PartialEq, Eq and Hash impls: de1c9d9.
• CFType Debug impl: c76799e.
• Forward other types' impls to CFType: 6d82a79.
• CFString conversions between &str: c76799e.
• Downcasting using CFTypeIDs: 3468e00.
• CFAllocator things?
  • https://github.com/yury/cidre/blob/530e3d3b1bf906ce8b045fbe073efd53cb8a1826/cidre/src/cf/base.rs#L410-L508
  • https://github.com/servo/core-foundation-rs/blob/49078e1f0f1e5fe395ddb01be426eef041646b30/core-foundation/src/data.rs#L42-L78
• CFNumber: 3e0e109.
• PartialOrd and Ord: ff0d74c.
• CFData: 5efb6eb.
• CFUUID: 933f49c.
• CFError: 4b630ef.
• CFDate: 3fe19b2.
• CFDictionary
• CFArray
• CFSet
• CFPropertyList