Fixes some things in azalea-protocol that have been bothering me for a while:

• Significantly simplify usage of the declare_state_packets macro (notably, packet ids are implicit now)
• Rename packet modules from like serverbound_keep_alive_packet to s_keep_alive
• Rename configuration and handshaking to config and handshake
• Remove the "Packet" suffix for packet structs (since the Clientbound/Serverbound prefix already indicates that it's a packet)
• Re-export all packet structs so they're accessible through like azalea_protocol::packets::game::ClientboundKeepAlive
• Rename ItemSlot to ItemStack for consistency with Mojmap
• Replace .get() with .into_variant() from a new Packet trait, which is used to avoid most uses of that function now.
• Rename read_from/write_into to azalea_read/azalea_write and McBufReadable/McBufWritable to AzaleaRead/AzaleaWrite
• Rename McBuf to AzBuf
• Add an attribute #[limit(number)] so strings can be limited to a certain size.

Based on the 1.21.4 PR since it's probably going to be merged in a few days anyways (diff). Also, these changes were mostly done with a lot of find-and-replace, so I may have messed up in some places without realizing. Please let me know if you notice something I broke or if you disagree with a change.

Some changes were inspired by @EightFactorial's FrogLight project <3

Closes #173