Simplify the syntax and representation of the separation logic empty heap constraint...

Simplify the syntax and representation of the separation logic empty heap constraint (#7268)

Since ~1 year ago, we insist that the location and data types of the separation logic heap are set upfront, analogous to the set-logic command. This means that the separation logic empty heap constraint does not need to be annotated with its types.

This makes SEP_EMP a nullary Boolean operator instead of binary predicate (taking dummy arguments whose types specify the heap type). It changes the smt2 extended syntax from (_ emp T1 T2) to sep.emp.