Support by-ref types

[sharwell]

The new ref struct feature in C# 7.2 results in several attributes applying to the compiled code. ILSpy should be updated to recognize these attributes and decompile the code to a ref struct instead.

As seen in dotnet/roslyn#22453, these attributes are:

• IsByRefLikeAttribute (applies for all ref struct)
• IsReadOnlyAttribute (applies for readonly ref struct)
• ObsoleteAttribute (emitted with the specific message hard-coded)

Let me know if you have any questions about this 😄

[siegfriedpammer]

Closing this, because support for the attributes was added long ago. #829 will be updated accordingly as soon as we've achieved test coverage and support for most use-cases.