Refactor chunked-encoding serialization:

New buffer sequence classes are provided to allow full
control over the serialization of chunk-encoded message
payloads:

* chunk_header

    A ConstBufferSequence representing the chunk header.
    It includes a hexadecimal-encoded size, an optional
    set of chunk extensions, and the trailing CRLF
    required to denote the end of the chunk header.

    This allows the caller to manually serialize the chunk
    body in one or more calls to a stream output function.
    The caller must also output an object of type `chunk_crlf`
    to the stream to indicate the end of the chunk body.

* chunk_crlf

    A small ConstBufferSequence that simply represents
    the two character sequence "\r\n" (CRLF). This is needed
    for the case where the caller wants to output a chunk
    body as a series of buffers (i.e. "chunking a chunk").

* chunk_body

    A ConstBufferSequence representing a complete chunk.
    This includes the size, an optional set of chunk extensions,
    a caller provided buffer containing the body, and the
    required CRLF that follows.

* chunk_final

    A ConstBufferSequence representing a final chunk. It
    includes an optional set of caller-provided field trailers

* chunk_extensions

    A container for building a set of chunk extensions to use
    during serialization. The use of the container is optional,
    callers may provide their own buffer containing a correctly
    formatted set of chunk extensions, or they may use their
    own convenience container which meets the requirements.

The basic_fields container is modified to allow construction
outside the context of a message. The container can be used
to provide trailers to `chunk_final`.

Actions Required:

* Remove references to ChunkDecorators. Use the new chunk-encoding
  buffer sequences to manually produce a chunked payload body in
  the case where control over the chunk-extensions and/or trailers
  is required.

include/beast/http/impl/fields.ipp

@@ -11,10 +11,11 @@
 #include <beast/core/buffer_cat.hpp>
 #include <beast/core/string.hpp>
 #include <beast/core/static_string.hpp>
+#include <beast/core/detail/buffers_ref.hpp>
 #include <beast/http/verb.hpp>
 #include <beast/http/rfc7230.hpp>
 #include <beast/http/status.hpp>
-#include <beast/http/detail/chunk_encode.hpp>
+#include <beast/http/chunk_encode.hpp>
 #include <boost/throw_exception.hpp>
 #include <stdexcept>
 #include <string>
@@ -140,18 +141,20 @@ public:
         }
     };
 
+    using view_type = buffer_cat_view<
+        boost::asio::const_buffers_1,
+        boost::asio::const_buffers_1,
+        boost::asio::const_buffers_1,
+        field_range,
+        chunk_crlf>;
+
     basic_fields const& f_;
-    boost::asio::const_buffer cb_[3];
+    boost::optional<view_type> view_;
     char buf_[13];
 
 public:
     using const_buffers_type =
-        buffer_cat_view<
-            boost::asio::const_buffers_1,
-            boost::asio::const_buffers_1,
-            boost::asio::const_buffers_1,
-            field_range,
-            boost::asio::const_buffers_1>;
+        beast::detail::buffers_ref<view_type>;
 
     reader(basic_fields const& f,
         unsigned version, verb v);
@@ -159,18 +162,28 @@ public:
     reader(basic_fields const& f,
         unsigned version, unsigned code);
 
+    reader(basic_fields const& f);
+
     const_buffers_type
     get() const
     {
-        return buffer_cat(
-            boost::asio::const_buffers_1{cb_[0]},
-            boost::asio::const_buffers_1{cb_[1]},
-            boost::asio::const_buffers_1{cb_[2]},
-            field_range(f_.list_.begin(), f_.list_.end()),
-            detail::chunk_crlf());
+        return const_buffers_type(*view_);
     }
 };
 
+template<class Allocator>
+basic_fields<Allocator>::reader::
+reader(basic_fields const& f)
+    : f_(f)
+{
+    view_.emplace(
+        boost::asio::const_buffers_1{nullptr, 0},
+        boost::asio::const_buffers_1{nullptr, 0},
+        boost::asio::const_buffers_1{nullptr, 0},
+        field_range(f_.list_.begin(), f_.list_.end()),
+        chunk_crlf());
+}
+
 template<class Allocator>
 basic_fields<Allocator>::reader::
 reader(basic_fields const& f,
@@ -188,12 +201,8 @@ reader(basic_fields const& f,
         sv = f_.get_method_impl();
     else
         sv = to_string(v);
-    cb_[0] = {sv.data(), sv.size()};
 
     // target_or_reason_ has a leading SP
-    cb_[1] = {
-        f_.target_or_reason_.data(),
-        f_.target_or_reason_.size()};
 
     buf_[0] = ' ';
     buf_[1] = 'H';
@@ -206,7 +215,15 @@ reader(basic_fields const& f,
     buf_[8] = '0' + static_cast<char>(version % 10);
     buf_[9] = '\r';
     buf_[10]= '\n';
-    cb_[2] = {buf_, 11};
+
+    view_.emplace(
+        boost::asio::const_buffers_1{sv.data(), sv.size()},
+        boost::asio::const_buffers_1{
+            f_.target_or_reason_.data(),
+            f_.target_or_reason_.size()},
+        boost::asio::const_buffers_1{buf_, 11},
+        field_range(f_.list_.begin(), f_.list_.end()),
+        chunk_crlf());
 }
 
 template<class Allocator>
@@ -234,16 +251,19 @@ reader(basic_fields const& f,
     buf_[10]= '0' + static_cast<char>((code / 10) % 10);
     buf_[11]= '0' + static_cast<char>(code % 10);
     buf_[12]= ' ';
-    cb_[0] = {buf_, 13};
 
     string_view sv;
     if(! f_.target_or_reason_.empty())
         sv = f_.target_or_reason_;
     else
         sv = obsolete_reason(static_cast<status>(code));
-    cb_[1] = {sv.data(), sv.size()};
 
-    cb_[2] = detail::chunk_crlf();
+    view_.emplace(
+        boost::asio::const_buffers_1{buf_, 13},
+        boost::asio::const_buffers_1{sv.data(), sv.size()},
+        boost::asio::const_buffers_1{"\r\n", 2},
+        field_range(f_.list_.begin(), f_.list_.end()),
+        chunk_crlf{});
 }
 
 //------------------------------------------------------------------------------