Some beginners are confused about the "extern" section, so I added an
explanation comment to all examples. Furthermore, using the UARTx
interrupts when UART is actually being used in the same example may be
confusing, so I changed them all to SSI0/QEI0.
covers
- initialization and configuration of the timer; this is now a responsibility of
the application author
- correctness of `Monotonic::now()` in `#[init]`
- safety of `Monotonic::reset()`
closes#251