add mrb_strlen_lit which makes _lit macros safer

strlen(3) + string literal is usually optimized but strlen(3) doesn't
check if its argument is really a string literal. This is important for
mruby's _static functions to which some _lit macros are expanded (string
literals have static storage). See comment for some additional info.

remove unnecessary parentheses

include/mruby/string.h

@@ -83,7 +83,7 @@ mrb_bool mrb_str_equal(mrb_state *mrb, mrb_value str1, mrb_value str2);
 mrb_value mrb_str_dump(mrb_state *mrb, mrb_value str);
 mrb_value mrb_str_cat(mrb_state *mrb, mrb_value str, const char *ptr, size_t len);
 mrb_value mrb_str_cat_cstr(mrb_state *mrb, mrb_value str, const char *ptr);
-#define mrb_str_cat_lit(mrb, str, lit) mrb_str_cat(mrb, str, (lit), sizeof(lit) - 1)
+#define mrb_str_cat_lit(mrb, str, lit) mrb_str_cat(mrb, str, lit, mrb_strlen_lit(lit))
 mrb_value mrb_str_append(mrb_state *mrb, mrb_value str, mrb_value str2);
 
 int mrb_str_cmp(mrb_state *mrb, mrb_value str1, mrb_value str2);