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)
It was supposed to say "wouldn't work on non 64 bit" :-/
Point being that there is no `Program Files (x86)` on all systems.