Rename MessageWriter to MessagePacker.

[Mark-Simulacrum]

Writers send data to the network (and the file MessagePacker does not do this).

[rousskov]

I suggest also renaming rawFoo() function in MessagePacker to packFoo() functions. This will complete the symmetry with parseFoo() methods in MessageParser.

Despite this renaming, I would leave the Header::_raw data member as is for now. It could be renamed to packed later, but I expect us to add some protocol version information to that data member so that HTTP/2 packers do not accidentally return pre-packed HTTP/1 headers. Until then, the raw/packed name disparity may alert developers of the potential raw usage problems.

[Mark-Simulacrum]

Fixed merge conflicts.