Compile a Rust version of MASM `auth__tx_rpo_falcon512`

[greenhat]

Open questions:

• Expose hashing into the Rust space (hmerge hmerge hmerge);
• Getting a public key from the account storage (we have low-level get_item(index: u8) -> Word);

MASM version is defined at:

https://github.com/0xMiden/miden-base/blob/e4912663276ab8eebb24b84d318417cb4ea0bba3/crates/miden-lib/asm/account_components/rpo_falcon_512.masm?plain=1

https://github.com/0xMiden/miden-base/blob/e4912663276ab8eebb24b84d318417cb4ea0bba3/crates/miden-lib/asm/miden/auth/mod.masm?plain=1

https://github.com/0xMiden/miden-base/blob/e4912663276ab8eebb24b84d318417cb4ea0bba3/crates/miden-lib/asm/miden/auth/rpo_falcon512.masm?plain=1

[bobbinth]

Getting a public key from the account storage (we have low-level get_item(index: u8) -> Word);

The low-level API should be fine for now. That is, we can get a word, and this word can be converted to the public key (the public key is just PubKey(Word).

Expose hashing into the Rust space (hmerge hmerge hmerge);

One option for this is to have something like miden::rpo::merge() function which could have the following signature:

pub fn merge(a: Word, b: Word) -> Word

Or it could be:

pub fn merge(values: [Word; 2]) -> Word