oceani: introduce type->prepare()

oceani: introduce type->prepare()

During parsing and analysis we don't want to "init" a
variable as that can allocate memory - when we add arrays,
we might not know yet how much memory.

So introduce 'prepare' to prepare a value - such that calling
free on it will work - without allocating.

'init' is then called when a variable is declared - unless something
is assigned to it instead.

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