Issue Description:

The NSSortDescriptor extension for working with KeyPath values is quite broken. The computed keyPath property relies on converting the String back into an AnyKeyPath by using a global table that is populated when converting the KeyPath into a String in the first place. The problem is, the String can't include the root type, so two different KeyPath values that have the same Obj-C keypath string will overwrite each other, and cause the NSSortDescriptor.keyPath property to return the wrong value.

Example:

import Foundation
class Foo: NSObject {
    @objc var name: String?
}
class Bar: NSObject {
    @objc var name: String?
}
let desc = NSSortDescriptor(keyPath: \Foo.name, ascending: true)
print(desc.keyPath == \Foo.name) // true
_ = NSSortDescriptor(keyPath: \Bar.name, ascending: true)
print(desc.keyPath == \Foo.name) // false
print(desc.keyPath == \Bar.name) // true

Besides equality, this also affects the behavior of the AnyKeyPath.appending(path:) method.

Personally, I think we should just do away with the keyPath property entirely. Barring that, I believe the correct solution here is to use Obj-C associated objects to associate the original KeyPath value with the NSSortDescriptor and do away with the global table.