add static-assert to sex

Expands to C11 _Static_assert keyword

tests/codegen.scm

@@ -259,4 +259,15 @@ compiles."
               "/* A 2D point. */"))
     (test-assert "and an enum docstring too"
       (emits? "(enum color \"RGB.\" (red green blue))"
-              "/* RGB. */"))))
+              "/* RGB. */")))
+
+  ;; `static-assert' is the keyword rather than the <assert.h> macro, so
+  ;; a static assertion costs no include. Mapped in `atom-to-fmt-c'
+  ;; because `unkebabify' alone would spell it `static_assert'.
+  (test-group "static-assert"
+    (test-assert "emits the C11 keyword"
+      (emits? (in-fn "(static-assert (== (sizeof int) 4) \"int is four bytes\")")
+              "_Static_assert(sizeof(int) == 4, \"int is four bytes\")"))
+    (test-assert "and not the header macro"
+      (not (emits? (in-fn "(static-assert (== (sizeof int) 4) \"x\")")
+                   "static_assert(")))))