Support Deserializing Http Dates (#86)

* Support Deserializing Http Dates

Deserializing HTTP dates is made slightly more complex because Smithy actually supports a format _not_ outlined by the RFC where the HTTP date
uses fractional seconds. For this reason (as well as dropping the chrono dependency in the future), `httpdate` was forked and incorporated into smithy-types. At a later date, we _may_ consider inlining this dependency into the crates that actually need it.

As a side change, I'm starting to refactor the way that custom serialization works via new-types. This enables generating composable serializers
that are much easier to autogenerate.