The API style is now more uniform: both are passed by mutable ref, and
only expose accessors. Additionally, the methods that were initially
accessed through the scheduler field are now directly implemented on
`Context`.
The build context is now passed as a mutable reference due to the need
to mutate data when adding a model.
Contains small unrelated cleanups and documentation improvements too.
The external_input example has been as well adapted and (at least
temporarily) simplifiedi/modified to remove the dependencies on
`atomic_wait` and `mio`.
