SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Switch to using additional overloads of sf::Packet, so we can eliminate
some of the messy code and just use the normal syntax for
BigEndianValue.

We can't avoid helper functions with u64 due to SFML's non-standard way
of defining 64-bit integer types.

Now that SFML's packet class can properly handle 64-bit values, we don't
need a helper function just to write values to the packets.

This adds the functionality of sending the host's save data (raw memory
cards, as well as GCI files and Wii saves with a matching GameID) to
all other clients. The data is compressed using LZO1X to greatly reduce
its size while keeping compression/decompression fast. Save
synchronization is enabled by default, and toggleable with a checkbox
in the NetPlay dialog.

On clicking start, if the option is enabled, game boot will be delayed
until all players have received the save data sent by the host. If any
player fails to receive it properly, boot will be cancelled to prevent
desyncs.