It is always implemented exactly the same way (as an uint8) so there is
no reason to implement it in each target separately.
This also makes it easier to add some documentation to it.
This commit refactors PWM support in the machine package to be more
flexible. The new API can be used to produce tones at a specific
frequency and control servos in a portable way, by abstracting over
counter widths and prescalers.
The `machine.Pin` type was a int8, which works fine but limits the
number of pin numbers to 127. This patch changes the type to uint8 and
changes NoPin to 0xff, which allows more pins to be used.
Some boards might not have that many pins but their internal
organization requires more pin numbers to be used (because it is
organized in pin ports and not all pins in a port have a physical
connection). Therefore the range of a int8 is too low to address these
higher pins.
This patch also has the surprising side effect of reducing binary size
in a number of cases. If there is a reduction it's usually just a few
bytes, with one outlier: the driver example amg88xx when compiled for
the pybadge board. I have not seen any increases in binary size.
Instead of configuring machine.I2C0, machine.I2C1, etc. statically,
allow the pins to be set using machine.I2CConfig. This will also
automatically configure the correct pin mode for each pin instead of
having to specify that manually.
