fix: conj 0-arg, conj onto nil (builds list), conj map into map

- (conj) -> []
- (conj nil x ...) builds a list (prepends): (conj nil 1 2) -> (2 1)
- conj a map value into a map/phm merges its entries ((conj {:a 0} {:b 1}) ->
  {:a 0 :b 1}); a [k v] vector/pair still adds one entry.

conj.cljc 14/6/5 -> 21/3/1. clojure-test-suite pass 3681->3691, errors 105->98.
spec: seq/conj-edge-cases (8). jpm test green.

src/jolt/core.janet

@@ -283,7 +283,16 @@
 # Collections
 # ============================================================
 
-(defn core-conj [coll & xs]
+# Is x a map value (for conj/merge semantics: conj-ing a map merges its entries)?
+(defn- map-value? [x]
+  (or (phm? x) (and (struct? x) (nil? (get x :jolt/type)))))
+
+(defn core-conj [& args]
+  (if (= 0 (length args)) (make-vec @[])        # (conj) -> []
+  (let [coll (first args) xs (tuple/slice args 1)]
+  (if (nil? coll)
+    # conj onto nil builds a list (prepends): (conj nil 1 2) -> (2 1)
+    (do (var result nil) (each x xs (set result (pl-cons x result))) result)
   (if (pvec? coll)
     (do (var result coll) (each x xs (set result (pv-conj result x))) result)
   (if (plist? coll)
@@ -303,20 +312,20 @@
         (if (phm? coll)
           (do
             (var result coll)
-            (var i 0)
-            (while (< i (length xs))
-              (let [pair (xs i)]
-                (set result (phm-assoc result (vnth pair 0) (vnth pair 1))))
-              (++ i))
+            (each x xs
+              (if (map-value? x)
+                # conj a map -> merge its entries
+                (each k (if (phm? x) (keys (phm-to-struct x)) (keys x))
+                  (set result (phm-assoc result k (if (phm? x) (phm-get x k) (in x k)))))
+                (set result (phm-assoc result (vnth x 0) (vnth x 1)))))
             result)
           (do
             (var result coll)
-            (var i 0)
-            (while (< i (length xs))
-              (let [pair (xs i)]
-                (set result (merge result {(vnth pair 0) (vnth pair 1)})))
-              (++ i))
-            result))))))))
+            (each x xs
+              (if (map-value? x)
+                (set result (merge result (if (phm? x) (phm-to-struct x) x)))
+                (set result (merge result {(vnth x 0) (vnth x 1)}))))
+            result)))))))))))
 
 (defn core-assoc [m & kvs]
   (cond

test/integration/clojure-test-suite-test.janet

@@ -18,7 +18,7 @@
 
 # Baseline: assertions Jolt currently passes across the suite. Raise as Jolt
 # improves so a regression (previously-passing assertion breaking) is caught.
-(def baseline-pass 3680)
+(def baseline-pass 3690)
 # A file is "clean" when it ran with zero failures AND zero errors.
 (def baseline-clean-files 45)
 # Per-file wall-clock budget (seconds). Normal files finish in well under 1s;

test/spec/sequences-spec.janet

@@ -118,3 +118,14 @@
   ["map a map"          "[1 nil 2]"      "(map {:a 1 :b 2} [:a :z :b])"]
   ["take-nth transducer" "[0 2 4 6 8]"   "(into [] (take-nth 2) (range 10))"]
   ["interpose transducer" "[1 :x 2]"     "(into [] (interpose :x) [1 2])"])
+
+# conj edge cases: 0-arg, conj onto nil (builds a list), conj a map into a map.
+(defspec "seq / conj edge cases"
+  ["conj no args"       "[]"        "(conj)"]
+  ["conj nil one"       "[3]"       "(conj nil 3)"]
+  ["conj nil many"      "[2 1]"     "(conj nil 1 2)"]
+  ["conj vector"        "[1 2 3]"   "(conj [1 2] 3)"]
+  ["conj list prepend"  "[0 1 2]"   "(conj '(1 2) 0)"]
+  ["conj map + map"     "{:a 0, :b 1}" "(conj {:a 0} {:b 1})"]
+  ["conj map + pair"    "{:a 0, :b 1}" "(conj {:a 0} [:b 1])"]
+  ["conj map merge wins" "{:a 2}"   "(conj {:a 0} {:a 1} {:a 2})"])