Http Prefix Header Support (#263)

* Http Prefix Header Support

The impetus of this diff is adding support for HttpPrefix headers. This ended up leading to a bunch of other changes:

1. operation.build() now always returns a result to account for inability to construct HTTP requests from certain inputs
2. HeaderName/HeaderValue is now created eagerly to give a better error message
3. httpPrefixHeaders are supported
4. The Dynamo movies example now uses the fluent client.

* Disable clippy lint of extra wrap

* Fix example

* Delete some dead code