refactor(ffi): Add Reason-Phrase API

This adds an internal ability to copy the HTTP/1 reason-phrase and place
it in the `http::Extensions` of a response, if it doesn't match the
canonical reason. This could be exposed in the Rust API later, but for
now it is only used by the C API.

src/ffi/mod.rs

@@ -28,7 +28,7 @@ mod io;
 mod task;
 
 pub(crate) use self::body::UserBody;
-pub(crate) use self::http_types::HeaderCaseMap;
+pub(crate) use self::http_types::{HeaderCaseMap, ReasonPhrase};
 
 pub const HYPER_ITER_CONTINUE: libc::c_int = 0;
 #[allow(unused)]