Sort namespaces in source files as per Microsoft .NET convention

[codemzs]

Parent issue #1697

StyleCop rules:
SA1210: Using directives must be ordered alphabetically by namespace.
SA1208: System using directives must be placed before other using directives.

[justinormont]

This is great; we should get ML.NET further into the conventions of .NET. Is there a list of them, or do they just follow StyleCop defaults?

cc: @danmosemsft