perf: Change underlying Elems to *const u8

[joncinque]

Problem

As part of the work in #29055 to refactor the Packet type, the GPU sigverify path needs to work for two different types, and not just Packet.

Summary of Changes

The underlying perf libs take uint8_t*, so make Elems consistent with that, which will also make it work for another Packet type.

Fixes #

[behzadnouri]

@joncinque
Seems like num field here is the number of Packets, not bytes.
So if elems does not have a known type, num becomes ambiguous.
e.g. how would the C code iterate over elems if their size is unknown.

[joncinque]

That's correct that num refers to the number of elements, and not their size.

If you look at the API that uses Elems, however, which is ed25519_verify_many and ed25519_sign_many, you'll see that we pass message_size, which defines the size of each packet. In fact, we currently pass in size_of::<Packet>() as u32 for that parameter. So the size is not defined by this type here.

You can even see the underlying function calls in solana-perf-libs starting at https://github.com/solana-labs/solana-perf-libs/blob/233c4457fd8d425da99cb3db46b2be8eb8b98997/src/cuda-ecc-ed25519/ed25519.h#L35, where they just use a uint_8 * underneath.