This PR adds a new `standard_policy` module within the retry module
that provides a batteries included policy to be used with the retry
middleware. The policy combines the `Budget` type and generic backoff
utlities from the `backoff` module to provide an easy to use policy
with good defaults.

This PR also includes a `StandardRetryPolicyBuilder` as well as two
new traits `IsRetryable` and `CloneRequest`. These each have blanket
impls for closures. The reason that this implementation breaks these
out into two different traits is to allow `tower-http` to provide
a custom `CloneRequest` implementation that will be able to clone
some sort of `ReplayBody` and let the user pass in the retry decision
implementation.

Ref #682

Co-authored-by: Eliza Weisman <[email protected]>