Task: use "Use" to represent theory use
up to this point, we used Clone declarations with an empty substitution to represent use of theories in tasks. The intention was to stress the fact that the imported declarations are physically present in the task and thus are followed by a "witness" Clone declaration (whereas a Use inside a theory acts rather as a pointer to follow). However, this encoding requires the clone substitution to cover every locally defined symbol: otherwise we might not be able to distinguish a use from a clone. Therefore, we had to clone even Pgoal propositions as Pskip, in order to keep the substitutions complete. This commit restricts the Clone declarations in tasks to actual theory cloning, and represents theory use with Use declarations. This hopefully makes the API more clear, and will allow us to abolish Pskip.