A [*FieldsWriter] provides a algorithm to obtain a sequence of buffers representing the complete serialized HTTP/1 header for a set of fields. The implementation constructs an instance of this type when needed, and calls into it once to retrieve the buffers.
* `a` is a value of type `W`. * `f` is a value of type `F`. * `v` is an `unsigned` value representing the HTTP version. * `c` is an `unsigned` representing the HTTP status-code. * `m` is a value of type [link beast.ref.boost__beast__http__verb `verb`].
The implementation calls this constructor to indicate that the fields being serialized form part of an HTTP request. The lifetime of `f` is guaranteed to end no earlier than after the `W` is destroyed.
The implementation calls this constructor to indicate that the fields being serialized form part of an HTTP response. The lifetime of `f` is guaranteed to end no earlier than after the `W` is destroyed.