WIP: Write PDBs to stream instead of file

This enables writing the PDB to a memory-stream when shimming the
file-system. Code mostly converted from Roslyn
(http://source.roslyn.codeplex.com/#Microsoft.CodeAnalysis/InternalUtilities/ComStreamWrapper.cs,5dbd4388f94fa102)
Minor not - don't use "with", just format like this:

    type X = 
        { ReadFile: string -> byte array option; 
          WriteFile: string -> Stream option }  
        static member ABC() = ...