hgweb: return content iterator instead of using write() callable

hgweb: all protocol functions have become generators Using the write() callable supplied by the start_response() call is frowned upon by the WSGI spec, returning an iterable over the content chunks is the recommended way. Be aware, though: returning many small chunks will slow down responses, because the server has to flush each chunk separately.

templater: make a template a string-only iterator