We've reached a point where we need five shared handlers
(tinyusb, micropython machine.Pin, twice PIO UART, encoders).
However, Pico SDK by default only provides four slots for handlers.

This commit works this around by dropping the use of shared IRQ
handlers in the PIO UART IRQ code. Encoder code requires them, so
I cannot drop it there.

It should also be possible to raise the number of shared handlers
by somehow defining PICO_MAX_SHARED_IRQ_HANDLERS via CMake.
We don't need this yet as with this change, we're back to
three out of four handlers.