Implemented selection of best user-defined conversion operator

According to §6.4.4 and §6.4.5

Daniel, can you please take a look at this? Especially I am wondering about the rather long comment in CSharpConversions.SelectOperator, where I had to write code which I cannot find any justification for in the spec.

Also, I do not understand the failing tests. I do not understand which part of the spec that causes them to fail, nor do I understand how to fix them.