rewrite sl_c_str as a function, will probably eventually do this for most pointless macros

slibs.h

@@ -141,12 +141,15 @@ typedef sl_vec(char) sl_string;
 
 #define sl_str_free(str) sl_vec_free(str)
 
-#define sl_c_str(str)         \
+const char* sl_c_str(sl_string str);
-  ({                          \
+
-    sl_vec_push((str), '\0'); \
+#ifdef SL_IMPLEMENTATION
-    (str).size--;             \
+const char* sl_c_str(sl_string str) {
-    (str).data;               \
+  sl_vec_push(str, '\0');
-  })
+  str.size--;
+  return str.data;
+}
+#endif
 
 void sl_append_c_str(sl_string *sl_str, const char *c_str);