Add MetadataOptimizerStep

We need to port existing Xamarin metadata optimizer to help us further with size reductions. ILLinker has now more information available about reflection scenario which could allow us to be more aggressive.

Original Step -- https://github.com/xamarin/xamarin-macios/blob/master/tools/linker/MonoTouch.Tuner/MetadataReducerSubStep.cs?rgh-link-date=2019-09-18T08%3A25%3A52Z

Proposed improvements
- Remove fields names. This is especially valuable for C# compiler generated names which are unnecessary long (e.g. `.field static assembly initonly class ''/'__StaticArrayInitTypeSize=256' '21244F82B210125632917591768F6BF22EB6861F80C6C25A25BD26DFB580EA7B'` and similar), more info at #1282
- Apply `StackTraceHiddenAttribute` more aggressive in libraries and strip all metadata for such methods (for example for `System.SR` methods)