Libcurl expects that headers are iterated in the same order that they
are recieved. Previously this caused curl tests 580 and 581 to fail.
This necessitated exposing a way to preserve the original ordering of
http headers.

SUMMARY OF CHANGES: Add a new data structure called OriginalHeaderOrder that
represents the order in which headers originally appear in a HTTP
message. This datastructure is `Vec<(Headername, multimap-index)>`.
This vector is ordered by the order which headers were recieved.
Add the following ffi functions:
- ffi::client::hyper_clientconn_options_set_preserve_header_order : An
     ffi interface to configure a connection to preserve header order.
- ffi::client::hyper_clientconn_options_set_preserve_header_case : An
     ffi interface to configure a connection to preserve header case.
- ffi::http_types::hyper_headers_foreach_ordered : Iterates the headers in
     the order the were recieved, passing each name and value pair to the callback.
Add a new option to ParseContext, and Conn::State called `preserve_header_order`.
This option, and all the code paths it creates are behind the `ffi`
feature flag. This should not change performance of response parsing for
non-ffi users.

CLOSES ISSUE: hyperium#2780

…ders_foreach_ordered

hyper_headers_foreach_ordered was merged into hyper_headers_foreach.
OriginalHeaderOrder docs were given examples
OriginalHeaderOrder has named members for better clarity.

BREAKING CHANGE: ffi::client::hyper_clientconn_options_new no longer
sets the http1_preserve_header_case connection option by default.
Users should now call
ffi::client::hyper_clientconn_options_set_preserve_header_order
if they desire that functionality

This cased the CI to fail