Add a consume paramter to request operations

[adriangb]

Currently if you want to parse a form without consuming the body you have to do:

await request.body()  # caches the body to memory
await request.form()  # parses the cached body as a form

It might be nice to add a Request.form(consume=False) API and a Request.json(consume=True) API to make this more intuitive.
Also for the JSON case there is no way to read a JSON body without duplicating it in memory, although JSON parsing is much easier than form parsing so it's not that big of a deal (it would be if it was the other way around).

[Kludex]

I'll close this as outdated. Correct me if wrong.

[adriangb]

I forgot about this. I still think it’s relevant. There’s been some requests for form not consuming the body which I’ve lost track of.