* readonly members warnings WIP

* Remove dead code

* Readonly members semantic checks. Warn on implicit copy of 'this'.

* Add to readonly ref readonly test

* Warn on implicit copies for get property access

* Don't warn on calls to base methods

* Add test for setter accesses in a readonly member

* Allow readonly setter accesses in readonly methods

* Test increment, compound assignment, and field-like events

* Fixes from feedback

* Add mutating readonly event accessors test

* Expand on readonly base members test

* Check for implicit copies in foreach/using/deconstruction scenarios

* Split base method tests into multiple tests for clarity

* Test remaining cases

* Fix whitespace in test

* Add ReadOnlyMethod_RefLocal

* Fixes from Fred's feedback