Support very large headers

This completely refactors how headers are hpack-encoded.

Instead of trying to be clever, constructing frames on the go
while hpack-encoding, we just make a blob of all the
hpack-encoded headers first, and then we split that blob
in as many frames as necessary.

src/proto/streams/send.rs

@@ -133,10 +133,6 @@ impl Send {
 
         Self::check_headers(frame.fields())?;
 
-        if frame.has_too_big_field() {
-            return Err(UserError::HeaderTooBig);
-        }
-
         let end_stream = frame.is_end_stream();
 
         // Update the state
@@ -270,10 +266,6 @@ impl Send {
             return Err(UserError::UnexpectedFrameType);
         }
 
-        if frame.has_too_big_field() {
-            return Err(UserError::HeaderTooBig);
-        }
-
         stream.state.send_close();
 
         tracing::trace!("send_trailers -- queuing; frame={:?}", frame);