This allows wmem to be used from other libraries, namely wsutil.
It is often the case that a funtion exists in wsutil and cannot
be used with a wmem scope, requiring some code duplication or
extra memory allocations, or vice-versa, code in epan cannot be
moved to wsutil because it has a wmem dependency.
To this end wmem is moved to wsutil. Scope management remains part
of epan because those scope semantics are specific to dissection.
