Updates System.Collections.Immutable and System.Metadata.Reader to versions 1.1.33-beta and 1.0.18-beta, respectively.
     Updates VSMEF to the latest build from VSPRO_1.

     A few code fixes:
     - ImmutableArrayInterop is gone
       - in product we used in a couple of places where we now just create a copy of the byte[] (MetadataWriter)
       - test infrastructure uses a workaround with explicitly laid out union to avoid copies of large metadata blobs
     - EditorTestApp class was incorrectly importing MEF components resulting in a cycle. The new VSMEF reports such errors. (changeset 1385613)