break out `unify-with-sequential*`

This is so we can reuse it in the experimental Datomic namespace.
update experimental Datomic support

Support unification of PersistentVector with datomic.Datom
remove root-var calls, not needed w/ update-dom
note differences from core.unify
typo
LOGIC-103: another `<=fd` non-termination bug

Add supporting fns `interval-<=` and `interval->=`. Remove silly logic
from `<=fdc` implementation, just use `interval-<=`.
add test for LOGIC-103
LOGIC-104. Changed the ::nom dom to a set.
`root-var` not needed