Fully-qualified method names in diagnostic messages

[kodek16]

Right now when a diagnostic refers to a method, it does so by only using its name. This gets confusing when traits and templates come into play, as it gets hard to distinguish which type the method belongs to. The solution would be to make methods aware of their "fully-qualified" names that include the whole path from the root namespace to the method name. An example of such name could be Foo::bar for a method bar of type Foo.

Need to consider #4 when designing the solution.