datalog: unification + substitution + 28 tests (Phase 2, 72/72)

dl-unify returns immutable extended subst dict (or nil). dl-walk
chases bindings, dl-apply-subst recursively resolves vars. Lists
unify element-wise so arithmetic compounds work too. dl-ground? and
dl-vars-of for safety analysis (Phase 3).

lib/datalog/conformance.conf

@@ -6,9 +6,11 @@ MODE=dict
 PRELOADS=(
   lib/datalog/tokenizer.sx
   lib/datalog/parser.sx
+  lib/datalog/unify.sx
 )
 
 SUITES=(
   "tokenize:lib/datalog/tests/tokenize.sx:(dl-tokenize-tests-run!)"
   "parse:lib/datalog/tests/parse.sx:(dl-parse-tests-run!)"
+  "unify:lib/datalog/tests/unify.sx:(dl-unify-tests-run!)"
 )