Currently, the timer uses a `Now` trait to abstract the source of time.
This allows time to be mocked out. However, the current implementation
has a number of limitations as represented by #288 and #296.
The main issues are that `Now` requires `&mut self` which prevents a
value from being easily used in a concurrent environment. Also, when
wanting to write code that is abstract over the source of time, generics
get out of hand.
This patch provides an alternate solution. A new type, `Clock` is
provided which defaults to `Instant::now` as the source of time, but
allows configuring the actual source using a new iteration of the `Now`
trait. This time, `Now` is `Send + Sync + 'static`. Internally, `Clock`
stores the now value in an `Arc<Now>` value, which introduces dynamism
and allows `Clock` values to be cloned and be `Sync`.
Also, the current clock can be set for the current execution context
using the `with_default` pattern.
Because using the `Instant::now` will be the most common case by far, it
is special cased in order to avoid the need to allocate an `Arc` and use
dynamic dispatch.
Mateusz Mikuła