Forbid shadowing of store entries in general

[tsani]

This is related to #28 but more general. Resolving this would resolve #28, so that issue is invalidated by this one. We should also forbid duplicate theorem names, LF constructor names, comp constructor names, etc.

This can be accomplished at the level of the store itself: each add function should check whether the name already exists and raise a DuplicateEntry exception.