Macro to make a ptrlen out of a string literal.

I'm quite surprised I haven't needed this for anything else yet. I
suppose if I had it, I could have written most of my ptrlen_eq_strings
in terms of it, and saved a lot of gratuitous runtime strlens.

misc.h

@@ -107,6 +107,12 @@ int string_length_for_printf(size_t);
 /* Derive two printf arguments from a ptrlen, suitable for "%.*s" */
 #define PTRLEN_PRINTF(pl) \
     string_length_for_printf((pl).len), (const char *)(pl).ptr
+/* Make a ptrlen out of a compile-time string literal. We try to
+ * enforce that it _is_ a string literal by token-pasting "" on to it,
+ * which should provoke a compile error if it's any other kind of
+ * string. */
+#define PTRLEN_LITERAL(stringlit) \
+    TYPECHECK("" stringlit "", make_ptrlen(stringlit, sizeof(stringlit)-1))
 
 /* Wipe sensitive data out of memory that's about to be freed. Simpler
  * than memset because we don't need the fill char parameter; also