Figure out what interface the Response object should expose for streaming data

[python-trio]

Figure out what interface the Response object should expose for streaming data

[sethmlarson]

Had more discussions about this, here's the results of that discussion:

• Having a Response.as_file() method which exposes a file-like object that acts exactly as a usual open() file is desirable for csv readers, data-frames, etc
• Something which has caused problems for urllib3 in the past is users seeing the interface of 'HTTPResponse' and noticing it inherits IOBase but doesn't exactly behave like a file. Let's not make that mistake again.
• The as_file() interface shouldn't close once all data is read just like a real file but should close the contained Response object after all data is read. This prevents Responses from being left un-closed but also gives an interface that places nice with interfaces expecting files.

Also discussed the usage of .text()/.data()/.json() as methods that load all data onto the Response.
There were questions about whether you should be able to call .text() and then maybe .text() again.

If we are one-shot only then you'd only be able to call a single "body" function on a response. However @pquentin brought up that this would make things annoying for debugging as you'd have to reissue requests to get another body.

Another thought was that if a response body is completely loaded into memory we could hold onto that body internally on the request to allow calling .text(), .json(), and .data() again if needed.