stmhal, timer: Remove obsolete global variable.
stmhal, servo: Rename calibrate method to calibration.

It should be a noun, since it supports getting and setting depending on
the number of arguments given.
stmhal: Make switch a class, to follow suit of all other modules.

Instead of pyb.switch() as a function, it's more consistent (with
respect to all the other modules and peripherals) to have
pyb.Switch() create a switch object.  This then generalises to having
multiple switches.  Call the object to get its state.  Use sw.callback
to set the callback function for when the switch is pressed.
stmhal, usrsw: Update comments for new switch API.