oceani: handle variable-sized arrays better.

An array with size set by a constant variable(!) might have a different
size each time the declaration is encountered.  So we need to
re-evaluate the size each time.
We currently re-evaluate the size only if it is zero.

So for numerical-constant sized arrays, evaluate size during parsing.
For other arrays, re-evaulate each time using a new prepare_type method.

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