adds bitflags to Packet.Meta

Instead of a separate bool type for each flag, all the flags can be
encoded in a type-safe bitflags encoded in a single u8:
https://github.com/solana-labs/solana/blob/d6ec103be/sdk/src/packet.rs#L19-L31

sdk/Cargo.toml

@@ -39,6 +39,7 @@ full = [
 [dependencies]
 assert_matches = { version = "1.5.0", optional = true }
 bincode = "1.3.3"
+bitflags = "1.3.1"
 bytemuck = { version = "1.7.3", features = ["derive"] }
 borsh = "0.9.0"
 base64 = "0.13"