* Currently all Inputs run each time through the loop, which could be a
  problem when an input is slow. It would be good if the @: operator could
  also rate limit when Inputs occur.

  Copilot does not let `extern` be limited, but the actual input operation
  is done by ardino-copilot's readInput, and extern then just uses the
  value it sets. So, make each Input add a trigger, that sets a flag
  variable, and in readInput just check if the flag is set.

  It should be possible to implement this w/o adding overhead when @: is
  not used on an Input. Either, avoid checking the flag in this case,
  or check that the C compiler can optimise away a check of a flag that
  always succeeds.

* Naming could be more FRP-like. readfrom and readvoltage
  are too imperative sounding. (Outputs were already fixed to be
  FRP-style.)

  A polymorphic read operation could work, assuming type inference is
  able to usually determine what kind of value is wanted to be read.

	a <- stream a1
	pin1 := a

  Looks like this would work: While the type of a could be Bool or Voltage,
  going to a pin that only supports digital IO implies it must be a Bool.
  (Need to check if the type checker is able to work this out!)

* analogReference()
  Takes one of a set of defines, which vary depending on board,
  so how to pass that through Copilot? Could write a C
  switch statement, on an int, and have the boade modules define
  effectively enums.
* readvoltage gets a raw ADC value; add a stream function to convert that
  to a floating point actual voltage, given also a stream that contains
  the reference voltage.
* delayMicroseconds
* pulses
* random numbers
* Wire library