fix(switch): add missing begin around switch impl so that it can used as an expression

Author: SuperFola

Switch.ark

@@ -22,5 +22,6 @@
 # @author https://github.com/SuperFola
 (macro switch (value case then ...cases) {
   (macro var ($gensym))
-  (let var value)
-  (_switch_impl var case then ...cases) })
+  {
+    (let var value)
+    (_switch_impl var case then ...cases) } })

tests/switch-tests.ark

@@ -36,4 +36,8 @@
   (switch (let n 1)
     0 (test:expect false)
     1 (test:expect true)
-    _ (test:expect false)) })
+    _ (test:expect false))
+
+  # (switch ...) should count as a single expression
+  (test:eq (+ 0 (switch 1 1 1)) 1)
+  (test:eq ((fun () (switch 1 1 1))) 1) })