Complete the subroute tracking

[andrewdavidmackenzie]

Accumulate the subroute of an IO used by direrent connections, store into the connection and then use in compatible_types() without trying a bad calculation of it there.

See TODOs in
fn find_function_destinations()

also TODO in
pub fn connect(&mut self, from_io: IO, to_io: IO, level: usize) -> Result<()> {
// are we selecting from a sub-route of an IO, such as an array index or element of output object?
// TODO this requires the accumulation of the subroute to be done during connection building