kernel: $cond/$when/$unless + 12 tests [nothing]

Standard Kernel control flow. $cond walks clauses in order with `else`
catch-all; clauses past the first match are NOT evaluated. $when/$unless
are simple guards. 12 tests, 242 total.

lib/kernel/runtime.sx

@@ -237,6 +237,70 @@
       ((or (nil? xs) (= (length xs) 0)) ys)
       (:else (cons (first xs) (knl-list-concat (rest xs) ys))))))
 
+;; $cond — multi-clause branch.
+;;   ($cond (TEST1 EXPR1 ...) (TEST2 EXPR2 ...) ...)
+;; Evaluates each TEST in order; first truthy one runs its EXPRs (in
+;; sequence) and returns the last; if no TEST is truthy, returns nil.
+;; A clause with TEST = `else` always matches (sugar for $if's default).
+(define knl-cond-impl
+  (fn (clauses dyn-env)
+    (cond
+      ((or (nil? clauses) (= (length clauses) 0)) nil)
+      (:else
+        (let ((clause (first clauses)))
+          (cond
+            ((not (list? clause))
+              (error "$cond: each clause must be a list"))
+            ((= (length clause) 0)
+              (error "$cond: empty clause"))
+            ((and (string? (first clause)) (= (first clause) "else"))
+              (knl-cond-eval-body (rest clause) dyn-env))
+            (:else
+              (let ((test-val (kernel-eval (first clause) dyn-env)))
+                (cond
+                  (test-val (knl-cond-eval-body (rest clause) dyn-env))
+                  (:else (knl-cond-impl (rest clauses) dyn-env)))))))))))
+
+(define knl-cond-eval-body
+  (fn (body dyn-env)
+    (cond
+      ((or (nil? body) (= (length body) 0)) nil)
+      ((= (length body) 1) (kernel-eval (first body) dyn-env))
+      (:else
+        (begin
+          (kernel-eval (first body) dyn-env)
+          (knl-cond-eval-body (rest body) dyn-env))))))
+
+(define kernel-cond-operative
+  (kernel-make-primitive-operative
+    (fn (args dyn-env) (knl-cond-impl args dyn-env))))
+
+;; $when COND BODY... — evaluate body iff COND is truthy; else nil.
+(define kernel-when-operative
+  (kernel-make-primitive-operative
+    (fn (args dyn-env)
+      (cond
+        ((< (length args) 1)
+          (error "$when: expects (cond body...)"))
+        (:else
+          (let ((c (kernel-eval (first args) dyn-env)))
+            (cond
+              (c (knl-cond-eval-body (rest args) dyn-env))
+              (:else nil))))))))
+
+;; $unless COND BODY... — evaluate body iff COND is falsy; else nil.
+(define kernel-unless-operative
+  (kernel-make-primitive-operative
+    (fn (args dyn-env)
+      (cond
+        ((< (length args) 1)
+          (error "$unless: expects (cond body...)"))
+        (:else
+          (let ((c (kernel-eval (first args) dyn-env)))
+            (cond
+              (c nil)
+              (:else (knl-cond-eval-body (rest args) dyn-env)))))))))
+
 (define kernel-quasiquote-operative
   (kernel-make-primitive-operative
     (fn (args dyn-env)
@@ -514,6 +578,9 @@
       (kernel-env-bind! env "$sequence" kernel-sequence-operative)
       (kernel-env-bind! env "$quote"    kernel-quote-operative)
       (kernel-env-bind! env "$quasiquote" kernel-quasiquote-operative)
+      (kernel-env-bind! env "$cond"   kernel-cond-operative)
+      (kernel-env-bind! env "$when"   kernel-when-operative)
+      (kernel-env-bind! env "$unless" kernel-unless-operative)
       (kernel-env-bind! env "eval" kernel-eval-applicative)
       (kernel-env-bind!
         env