Set server response headers (#1086)

This commit adds support for the `httpHeaders` and `httpPrefixHeaders`
when applied to error shapes and operation output shapes to the server
implementation.

The presence of response headers is now asserted against in server
protocol tests.

Functions that set headers in requests that were used by the client have
been refactored into `HttpBindingGenerator.kt`, since they are useful
for the server to set headers in responses.

This commit also makes error shapes be serialized in JSON responses with
a `__type` field in the body, in favor of setting the `X-Amzn-Errortype`
header, as recommended by the specs of all AWS protocols.

This commit also removes the generation of operation structs for the
server in `ProtocolGenerator.kt`, since they are not useful for the
server implementation.

Closes #1071 #1075.