In summary, this commit modifies 'Responder' so that:
* ..it is no longer 'async'. To accommodate, the 'sized_body' methods
in 'Response' and 'ResponseBuilder' are no longer 'async' and accept
an optional size directly. If none is supplied, Rocket will attempt
to compute the size, by seeking, before writing out the response.
The 'Body' type was also changed to differentiate between its sized
'Seek' and chunked body variants.
* ..'&Request' gains a lifetime: 'r, and the returned 'Response' is
parameterized by a new 'o: 'r. This allows responders to return
references from the request or those that live longer.
The attribute is applied everywhere it can be across the codebase and is
the newly preferred method for launching an application. This commit
also makes '#[rocket::main]` stricter by warning when it is applied to
functions other than 'main'.
Sergio Benitez
Jeb Rosen