CountDown timer API should be configured with a time period, not a frequency

[dzimmanck]

The builder methods (start and start_countdown) accept a variable called timeout, which must be convertible into Hertz. This does not make sense to me. A Countdown should "count down from X" where X is a time duration, not a frequency. This leads to non-sensical looking code like:

let frequency = timeout.into.0

Also, it means you loose A LOT of precision on the timer. You can no longer precisely control the timer delay in increments of cycles due to this division.

let ticks = self.clk.0 / frequency;

[burrbull]

Related to rust-embedded/embedded-hal#201

[dbrgn]

Related to #235.

Related to rust-embedded/embedded-hal#201

Is there any reason why this couldn't / shouldn't be changed in this HAL without waiting for e-h to specify a unified standard?

[burrbull]

#381