Counting at 1 kHz, 32 bits for counting ticks is not enough to ensure
monotonicity for more than 50 days. Add a feature to change the backing
storage to 64 bits.
Some hals implement traits for embedded-hal version `=1.0.0.alpha.<not 9>`, which is
explicitly incompatible with the version `=1.0.0.alpha.9` which embedded-hal-async
depends on. Making the dependency optional allows downstream projects to include
rtic-monotonic without requiring that all of their other libraries also implement
that specific version of embedded-hal 1.0