Swift additions

These patches add some Swift convenience, such as initializing an XPCBool from a Swift Bool like so: `var anXpcBool: XPCBool = true`. Or setting an XPCString in a similar fashion: `var anXpcString: XPCString = "This is a string."`

I also added support for using `for ... in ...` support to the XPCArray class. Attempts for doing the same with the XPCDictionary class were stymied due to the lack of getting a list of dictionary keys.