Parameterize Scorer by a Time trait

Parameterize Scorer by a Time trait

Scorer uses time to determine how much to penalize a channel after a
failure occurs. Parameterizing it by time cleans up the code such that
no-std support is in a single AlwaysPresent struct, which implements the
Time trait. Time is implemented for std::time::Instant when std is
available.

This parameterization also allows for deterministic testing since a
clock could be devised to advance forward as needed.