Of_packed_array(P)
creates a packed array of packed arrays. The representation is a
P.t
and packed array of indices into it which point to the beginning of each inner
array.
This function only needs implementation if t
exposed to be a polymorphic variant.
Despite what the type reads, this does *not* produce a function after reading;
instead it takes the constructor tag (int) before reading and reads the rest of the
variant t
afterwards.