Function at_serialized_capacity 

pub(crate) fn at_serialized_capacity<'a, A>(borrow: &A) -> bool
where
    A: AsBytes<'a>,

Returns true once the serialized size of borrow is within 10% of the next SHIP_WORDS boundary.

Same heuristic as ColumnBuilder::push_into; lifted out so the merger and the SizableContainer impl agree on the ship signal.