js-on-sx: switch/case/default/break

Parser: jp-parse-switch-stmt + jp-parse-switch-cases + jp-parse-switch-body.
AST: (js-switch discr (("case" val body) ("default" nil body) ...)).

Transpile: wraps body in (call/cc (fn (__break__) ...)). Each case
clause becomes (when (or __matched__ (js-strict-eq discr val))
(set! __matched__ true) body). Fall-through works naturally via
__matched__. Default appended as (when (not __matched__) body).

363/365 unit (+6), 148/148 slice unchanged.

lib/js/parser.sx

@@ -1015,6 +1015,63 @@
         ((e (jp-parse-assignment st)))
         (do (jp-eat-semi st) (list (quote js-throw) e))))))
 
+(define
+  jp-parse-switch-stmt
+  (fn
+    (st)
+    (jp-advance! st)
+    (jp-expect! st "punct" "(")
+    (let
+      ((disc (jp-parse-assignment st)))
+      (jp-expect! st "punct" ")")
+      (jp-expect! st "punct" "{")
+      (let
+        ((cases (list)))
+        (jp-parse-switch-cases st cases)
+        (jp-expect! st "punct" "}")
+        (list (quote js-switch) disc cases)))))
+
+(define
+  jp-parse-switch-cases
+  (fn
+    (st cases)
+    (cond
+      ((jp-at? st "punct" "}") nil)
+      ((jp-at? st "keyword" "case")
+        (do
+          (jp-advance! st)
+          (let
+            ((val (jp-parse-assignment st)))
+            (jp-expect! st "punct" ":")
+            (let
+              ((body (list)))
+              (jp-parse-switch-body st body)
+              (append! cases (list "case" val body))
+              (jp-parse-switch-cases st cases)))))
+      ((jp-at? st "keyword" "default")
+        (do
+          (jp-advance! st)
+          (jp-expect! st "punct" ":")
+          (let
+            ((body (list)))
+            (jp-parse-switch-body st body)
+            (append! cases (list "default" nil body))
+            (jp-parse-switch-cases st cases))))
+      (else (error "switch: expected case or default")))))
+
+(define
+  jp-parse-switch-body
+  (fn
+    (st body)
+    (cond
+      ((jp-at? st "punct" "}") nil)
+      ((jp-at? st "keyword" "case") nil)
+      ((jp-at? st "keyword" "default") nil)
+      (else
+        (begin
+          (append! body (jp-parse-stmt st))
+          (jp-parse-switch-body st body))))))
+
 (define
   jp-parse-try-stmt
   (fn
@@ -1055,6 +1112,7 @@
       ((and (jp-at? st "keyword" "async") (= (get (jp-peek-at st 1) :type) "keyword") (= (get (jp-peek-at st 1) :value) "function"))
         (do (jp-advance! st) (jp-parse-async-function-decl st)))
       ((jp-at? st "keyword" "function") (jp-parse-function-decl st))
+      ((jp-at? st "keyword" "switch") (jp-parse-switch-stmt st))
       (else
         (let
           ((e (jp-parse-assignment st)))