This patch integrates pldb into core.logic, using the technique suggested by David Nolen of adding the pldb db to the substitution metadata. This worked really well. I've tested this integration against the threatgrid codebase, and with the exception of a couple edge cases where we were using facts-for directly for testing, it is a drop in replacements for pldb, requiring only the namespace change of pldb -> clojure.core.logic.pldb.
I moved logic-dbs into the clojure.core.logic so that they could automatically be included in the various run macros and the existing pldb/with-db bindings. I've also added run-db macros that allow explicit declarations where that is desirable. This seems reasonable to me, but I'm open to suggestions.