Move some LINQ helpers from CodeGenerator intoSugar
List<T> should be  sequence on all platforms
Workaround for [70126: Can't mix negative values and Uint in "if" expression, even if target is signed]
Made PreBuild event conditional to only run on Windows
ISequence<T>.Contains<T> fixes for Echoes
Optimization for constructor List<T>(anArray: array of T);