feature(ffi): add connection option to preserve header order (#2798)

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.
- 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 #2780

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

capi/include/hyper.h

@@ -355,6 +355,22 @@ void hyper_clientconn_free(struct hyper_clientconn *conn);
  */
 struct hyper_clientconn_options *hyper_clientconn_options_new(void);
 
+/*
+ Set the whether or not header case is preserved.
+
+ Pass `0` to allow lowercase normalization (default), `1` to retain original case.
+ */
+void hyper_clientconn_options_set_preserve_header_case(struct hyper_clientconn_options *opts,
+                                                       int enabled);
+
+/*
+ Set the whether or not header order is preserved.
+
+ Pass `0` to allow reordering (default), `1` to retain original ordering.
+ */
+void hyper_clientconn_options_set_preserve_header_order(struct hyper_clientconn_options *opts,
+                                                        int enabled);
+
 /*
  Free a `hyper_clientconn_options *`.
  */