[SoM] Need a Foundation assembly

[stephen-hawley]

There are types in libswiftFoundation.dylib that aren't the Xamarin namespaces, for example Foundation.Date which is a swift bridge to NSDate (which also exists - they're two different types).

As such, we should have another set of assemblies and libraries that are built like XamGlue that include the manual bindings for those types.

Currently SwiftDate is part of XamGlue, but it doesn't really belong there and would need to be moved and the pinvoke entry points updates etc etc.