reinstate function scTermF in SharedContext interface
reinstate function scTermF in SharedContext interface

(partially reverts 5c3bb8b897eaa76a58f9248d3df55a536acfe5b1;
it seems the interface needs low-level functions after all.)

GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script