Support tokio::time::Instant in SystemClock

[olix0r]

tokio::time::Instant is a special Instant type that supports mocking
for tests (via tokio::time::pause). Furthermore, it avoids
panics in Instant arithmetic.

When using tokio::time::Instant, there's no need for a dependency on
the instant crate.

This change:

• makes the instant crate an optional dependency, enabled by default;
• uses tokio::time::Instant when the tokio_1 feature is enabled and
  instant is not; and
• uses std::time::Instant when neither of these features are enabled.

The type is exposed publicly via backoff::Instant as a convenience.

This change is backwards-compatible and does not alter the default
behavior.

[nightkr]

I think it's pretty confusing to start mixing type aliases around like this based on what features are enabled. This PR would make it so that backoff::Instant == std::time::Instant unless the feature set matches tokio && !instant.

I'd rather pick one of:

1. Make backoff::Instant a newtype that implements the appropriate From instances for whatever features are enabled
2. Introduce a new TokioClock type users would use instead of SystemClock

[olix0r]

I think it's pretty confusing to start mixing type aliases around like this based on what features are enabled. This PR would make it so that backoff::Instant == std::time::Instant unless the feature set matches tokio && !instant.

Yeah, I don't disagree; though I'll note that the instant crate is already playing these games (albeit depending on platform, instead of feature flag).

1. Make backoff::Instant a newtype that implements the appropriate From instances for whatever features are enabled
2. Introduce a new TokioClock type users would use instead of SystemClock

In both of these cases, I think all of the Instants would have to be converted to std::time::Instant -- which is OK, but we'd need to audit the code for uses of elapsed/duration_since/sub (which all can panic) to make sure we're properly guarding against cases that can panic.

[olix0r]

This seems less relevant now that Rust 1.60 has removed panics from Duration operations