packet: Use Bytes instead of an array in Packet

The reason behind the switch is that `Packet` gets cloned in multiple
places. `Bytes` provides a zero-copy abstraction, where calling
`clone()` doesn't make an actual copy of the underlying data and all
instances point to the same memory.

However, the old layout containing a sized array is convenient for CUDA.
To not break CUDA support, this change introduces a new struct called
`PacketArray`, which a `Packet` can be converted into just before
calling GPU-based sigverify. That requires a copy, ideally just one.

At the same time, CPU-based sigverify and all the other components are
going to benefit from zero-copy properties of `Bytes`.

Cargo.toml

@@ -140,6 +140,7 @@ bs58 = { version = "0.5.1", default-features = false }
 bv = "0.11.1"
 bytemuck = "1.21.0"
 bytemuck_derive = "1.8.1"
+bytes = { version = "1.10", features = ["serde"] }
 cfg_eval = "0.1.2"
 chrono = { version = "0.4.39", default-features = false }
 console = "0.15.10"

packet/Cargo.toml

@@ -12,6 +12,7 @@ edition = { workspace = true }
 [dependencies]
 bincode = { workspace = true, optional = true }
 bitflags = { workspace = true }
+bytes = { workspace = true }
 cfg_eval = { workspace = true, optional = true }
 serde = { workspace = true, optional = true }
 serde_derive = { workspace = true, optional = true }