+ don't allocate space if we're passed an in-memory array as either
a source or a destination of split/combine

+ fix up the split_vec/combine_vec combinatino so that they both
work a bit better now - still some work needed here, though

+ free the internal s4 structure as part of the combine/combine_vec
operation, rather than doing it in ssss_end(). We know at that stage
that we have allocated it, and the principle of freeing memory in
the same place in the calling stack as it was allocated still applies

+ rip out parts of the initial comment which aren't really accurate
any more - i'm not sure there's that much of the original code left

(agc)