Convert LINQ fluent syntax to query syntax

The reverse operation of https://github.com/icsharpcode/NRefactory/pull/206

Some of the code was based on ILSpy's. Please let me know if the copyright header format is not the correct one so I can correct it. Obviously, I'm willing to release my changes under NRefactory's open-source license.

As far as my testing goes, I believe this covers all use-cases. Please let me know if you find any edge-case that I missed in my testing.

Also note that my changes are *NOT* ready to be merged back to ILSpy. I have performed no testing whatsoever to ensure it works in its original context and I strongly suspect it does not.