Implement Time-Keeping

[Cryptjar]

This PR adds:

• TimingCircuit trait to abstract over various hardware timers that are eligible for time-keeping.
• attach_timing_circuit_interrupt macro, which allows attaching timer interrupts in a somewhat generic manor.
• Timepiece trait and impl_timepiece macro, that users will use to "prepare" a hardware timer (i.e. a TimingCircuit) for the time-keeping facilities (i.e. the Choronometer). "Preparing" means here that the user defines the configuration (e.g. hardware, resolution) and the macro will attach a corresponding timer interrupt handler.
• Choronometer, a struct that wraps a Timepiece (which in turn wraps a TimingCircuit), initializes the corresponding hardware timer, and implements the embedded_time::Clock trait with microseconds precision.
• StaticChronometer a statically accessible embedded_time::Clock implementation that yields milliseconds precision.

For a quick peek, see the uno-time example

TODOs:

• implement TimingCircuit on all eligible hardware Timers
  • how to support 16-bit timers such as TC1 on Uno? (see No ctc function for atmega328p's WGM1_W avr-device#96)
  • should we support Timer Type B? (such as atmega4809, attiny202, attiny816)
• add an Arduino Mega example
• decide on what names to use
• polish docs

Fixes #256