Support UUID fields directly in IDL

Dealing with ByteString incurs a copy cost and for something as simple as GUID we might benefit from explicit 128-bit representation directly mappable to a [Guid constructor](https://msdn.microsoft.com/en-us/library/ms131157%28v=vs.110%29.aspx?cs-save-lang=1&cs-lang=fsharp#code-snippet-1).
