Change #process_code to take two params.

Instead of just returning the hash, process_code will now add fields
To the supplied context hash, with implementation in MemberTag covering
the base case and subclasses adding their extra logic on top of that.

Transformation of member name from "foo.bar.baz" to "baz" now
happens within MemberTag#process_code, while Class#process_code leaves
the name as is.

The #process_code now gets called for every time - both when the member
type was detected correctly and when it was not.