Make naming more consistent

[heaths]

Based on feedback from the architect review and some of our own conversations over time (see also, "naming is hard"), we've decided to make the following type name changes:

Description	Sync	Async
Main API	Response	AsyncResponse
Contained by parent	RawResponse	BufResponse AsyncRawResponse
Contained by parent	ResponseBody	BufResponseBody AsyncResponseBody

Make it clear in the description as well that this is based more on Response : AsyncResponse :: Iterator : AsyncIterator (in std), not .NET's pattern of slapping "Async" on the end (methods) or beginning (some types) of things.

Additionally, Response on down will have both borrowed ref getters and ownership gettings i.e., body(&self) -> &ResponseBody and into_body(self) -> ResponseBody because all data is available: no longer do we have to asynchronously collect all the bytes. We'll effectively rename the existing into_body(self) -> Result<T> to into_model(self) -> Result<T> based on architect review feedback and our own past conversations about this naming. This will be a fairly significant breaking change to customers, which is why we figured we'd break it all now in one shot.

AsyncResponse on down will still require taking ownership, but we also should also make sure we have consistent into_body(self) or similar patterns.