core-test: match new parallel invocation error exception class
Parallelize fold-op-into-worlds and possible-worlds via reducers
Models now return (knossos.core/inconsistent) instead of throwing.

Performance tests revealed that stacktrace generation dominated in
verification of broad, mostly-invalid histories. This is significantly
faster.

Also eliminates keep-singular, pkeep-singular, and friends; their logic
is folded into possible-worlds now.