hgweb: create dedicated type for WSGI responses We have refactored the request side of WSGI processing into a dedicated type. Now let's do the same thing for the response side. We invent a ``wsgiresponse`` type. It takes an instance of a request (for consulation) and the WSGI application's "start_response" handler. The type basically allows setting the HTTP status line, response headers, and the response body. The WSGI application calls sendresponse() to start sending output. Output is emitted as a generator to be fed through the WSGI application. According to PEP 3333, this is the preferred way for output to be transmitted. (Our legacy ``wsgirequest`` exposed a write() to send data. We do not wish to support this API because it isn't recommended by PEP 3333.) The wire protocol code has been ported to use the new API. Differential Revision: https://phab.mercurial-scm.org/D2775