* Add IsDeclaredReadOnly and IsEffectivelyReadOnly to public API

* Fix build. Add simple SemanticModel test.

* Fix Roslyn users of IMethodSymbol. Expand semantic model test.

* Fix visibility of MethodSymbol.vb members

* Implement ReadOnly method API in CodeGenerationConstructedMethodSymbol

* Cleanup usings and change NotImplementedException to ExceptionUtilities.Unreachable

* Simplify down to IMethodSymbol.IsReadOnly

* Fix IsReadOnly API for reduced extension methods. Test ref readonly ref
properties.

* Ensure that non-reduced extension methods are never 'readonly'

* Update doc

* Reduced extension methods are not declared readonly, they're effectively readonly.

* Remove unused copy of extension methods test source