common: tal_dup_talarr() helper.

This is a common thing to do, so create a macro.

Unfortunately, it still needs the type arg, because the paramter may
be const, and the return cannot be, and C doesn't have a general
"(-const)" cast.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/utils.h

@@ -56,6 +56,17 @@ void clear_softref_(const tal_t *outer, size_t outersize, void **ptr);
 #define tal_arr_remove(p, n) tal_arr_remove_((p), sizeof(**p), (n))
 void tal_arr_remove_(void *p, size_t elemsize, size_t n);
 
+/**
+ * The comon case of duplicating an entire tal array.
+ *
+ * A macro because we must not double-evaluate p.
+ */
+#define tal_dup_talarr(ctx, type, p)					\
+	((type *)tal_dup_talarr_((ctx), tal_typechk_(p, type *),	\
+				 TAL_LABEL(type, "[]")))
+void *tal_dup_talarr_(const tal_t *ctx, const tal_t *src TAKES,
+		      const char *label);
+
 /* Use the POSIX C locale. */
 void setup_locale(void);