Fix memory leaks in FileSystemTypeProviders

The file system type providers were implementing IDisposable themselves, when the underlying class already implemented it.  This causes a memory leak as the AppDomain's `AssemblyResolve` event handler was never unsubscribed.

Updated to latest ProvidedTypes.fs/fsi from the TypeProviderStarterPack, and used the `Disposing` event to handle the cleanup properly. For  details, refer to https://github.com/fsprojects/FSharp.TypeProviders.StarterPack/pull/23  (This is effectively the same issue described there)