The message class now behaves like a pair with respect to the construction
of the body and headers. Additional constructors allow construction of
just the body portion from a tuple, leaving the headers default
constructed.
Previous constructors are removed as they were a notational convenience
for assembling HTTP/1 requests and responses. They are not necessary
as this library aims at library writers and not end users.
The version field is moved into message_v1, all public interfaces
are reworked to identify HTTP/1 wire format operations (suffix "_v1")
versus general HTTP.
Vinnie Falco