Function capnp::serialize::compute_serialized_size_in_words

pub fn compute_serialized_size_in_words<A>(message: &Builder<A>) -> usize where
    A: Allocator, 

Returns the number of (8-byte) words required to serialize the message (including the segment table).

Multiply this by 8 (or std::mem::size_of::<capnp::Word>()) to get the number of bytes that write_message() will write.