datalog: dl-magic-query driver (204/204)

End-to-end magic-sets entry point. Given (db, query-goal):
  - copies the caller's EDB facts (relations not headed by any
    rule) into a fresh internal db
  - adds the magic seed fact
  - adds the rewritten rules
  - saturates and runs the query
  - returns the substitution list

Caller's db is untouched. Equivalent to dl-query for any
fully-stratifiable program; intended as a perf alternative on
goal-shaped queries against large recursive relations.

2 new tests: equivalence to dl-query on chain-3 ancestor, and
non-mutation of the caller's db (rules count unchanged).

lib/datalog/tests/magic.sx

@@ -196,6 +196,34 @@
               (len (dl-relation db "ancestor")))))
         6)
 
+      ;; dl-magic-query: end-to-end driver, doesn't mutate caller's db.
+      (dl-mt-test! "magic-query equivalent to dl-query"
+        (let
+          ((db (dl-program
+                 "parent(a, b). parent(b, c). parent(c, d).
+                  ancestor(X, Y) :- parent(X, Y).
+                  ancestor(X, Z) :- parent(X, Y), ancestor(Y, Z).")))
+          (let
+            ((semi (dl-query db (list (quote ancestor) (quote a) (quote X))))
+              (magic (dl-magic-query
+                       db (list (quote ancestor) (quote a) (quote X)))))
+            (= (len semi) (len magic))))
+        true)
+
+      ;; magic-query doesn't mutate caller db.
+      (dl-mt-test! "magic-query preserves caller db"
+        (let
+          ((db (dl-program
+                 "parent(a, b). parent(b, c).
+                  ancestor(X, Y) :- parent(X, Y).
+                  ancestor(X, Z) :- parent(X, Y), ancestor(Y, Z).")))
+          (let
+            ((rules-before (len (dl-rules db))))
+            (do
+              (dl-magic-query db (list (quote ancestor) (quote a) (quote X)))
+              (= rules-before (len (dl-rules db))))))
+        true)
+
       (dl-mt-test! "magic-rewritten finds same answers"
         (let
           ((rules