oceani: pass a destination buffer into interp_exec to receive large result.

To handle assignment from large objects - particularly a structure
returned by a function - we cannot just pass back a 'struct value'.
Instead we need to pass in a sufficiently large buffer, and have
the value producer copy into it.

This patch passes in a 'dest' and 'dtype' for this purpose.  It is not
yet used to full potential.

Signed-off-by: NeilBrown <neil@brown.name>