Fixes for mcs

• LINQ
• Delegate construction

Misc

• Updated pattern detection for changes in Roslyn 2.7
• Refactored ILSpy.AddIn

Improvements in TypeSystem

• Lazy-loading for nested types (needs testing)
• Add metadata tokens to type system
• Remove dead code

Bug Fixes for Roslyn

• dotnet/roslyn#25251 (Decompilation should simplify "type" usage)
• dotnet/roslyn#25246 (Decompilation cannot decompile xUnit's Assert.All)
• #1095 (C# decompilation, for flags enums always use hex prefix)