This is really useful for anyone who wishes to use the steven_protocol
crate and write tests that compare packets to expected values.

For users of the crate, it's important to be able to set this if
they are not going through the `Conn` class.

Ideally, the protocol version would be plumbed through every
`Serializable` impl, but that's a much bigger refactor.

Previously, the Packet enum was a whopping 4,264 bytes in size (over a page!).
Not only is this just bad for performance (lots of memmoving), but I think it was
also causing weird stack overflow segfaults on my machine.

Currently, the `hyper` crate (dependency of `reqwest`) causes linker errors when
you attempt to compile a consumer of `steven_protocol` as a dylib. Compiling as
a dylib is not uncommon for quick iteration; notably popularized by the Bevy
game engine.

See rust-lang/rust#82151 (comment).