Add introspection feature to allow array subsets

This assumes your code knows to check for empty slots, e.g. if they
are zero-initialized.

src/Introspection.cake

@@ -1288,7 +1288,11 @@
                                                (addr value-argument-end))
            ;; (print-string-range value-argument-start value-argument-end true)
            (unless (and value-argument-start value-argument-end)
-             (fprintf stderr "error: field %s expected %d elements but found only %d\n"
+             (when (metadata-field-has-tag current-field "'array-allow-subset")
+               (set state state-read-member-name)
+               ;; introspect-field-dispatch is a loop for all fields
+               (break))
+             (fprintf stderr "error: field %s expected %d elements but found only %d. Add 'array-allow-subset to allow this.\n"
                       (path current-field > name) (type-cast (path current-field > count) int)
                       dispatch-value-index)
              (return false))))