Mofify IMP so that the K "followed by" arrow, ~>, does not explicitly
occur in the definition (it currently occurs in the semantics of
sequential composition).

Hint: make sequential composition strict(1) or seqstrict, and have
statements reduce to "{}" instead of "."; and don't forget to make
"{}" a KResult (you may need a new syntactic category for that, which
only includes "{}" and is included in KResult).
