make sure that important trait methods are easy to find

[milyin]

This update #688 moves several key functions of Session to separate trait. Need to make sure that it's easy to navigate to this functionality in the documentation.
This is a problem in rust autogenerated documentation: methods of structure are considered important and visible immediately, but trait implementations are deep buried in lots of helping traits like Deref, Cloneand Display. If this happens, need to make sure that key traits like SessionDeclarations are linked from the structure description.

[Mallets]

Closed by #1193