Prior to this commit, all forward outcomes resulted in a 404. This
commit changes request and data guards so that they are able to provide
a `Status` on `Forward` outcomes. The router uses this status, if the
final outcome is to forward, to identify the catcher to invoke.
The net effect is that guards can now customize the status code of a
forward and thus the error catcher invoked if the final outcome of a
request is to forward.
Resolves#1560.
This modifies the 'IoHandler::io()' method so that it takes a
'Pin<Box<Self>>', allowing handlers to move internally and assume that
the data is pinned.
The change is then used in the 'ws' contrib crate to allow 'FnOnce'
handlers instead of 'FnMut'. The net effect is that streams, such as
those crated by 'Stream!', are now allowed to move internally.
