Use FCMP implementation of BP+ in monero-serai (#344)

* Add in an implementation of BP+ based off the paper, intended for clarity and review

This was done as part of my work on FCMPs from Monero, and is copied from https://github.com/kayabaNerve/full-chain-membership-proofs

* Remove crate structure of BP+

* Remove arithmetic circuit code

* Remove AC/VC generators code

* Remove generator transcript

Monero uses non-transcripted static generators.

* Further trimming of generators

* Remove the single range proof

It's unused by Monero and accordingly unhelpful.

* Work on getting BP+ to compile in its new env

* Correct BP+ folder name

* Further tweaks to get closer to compiling

* Remove the ScalarMatrix file

It's only used for AC proofs

* Compiles, with tests passing

* Lock BP+ to Ed25519 instead of the generic Ciphersuite

* Resolve most warnings in BP+

* Make existing bulletproofs test easier to read

* Further strip generators

* Swap G/H as Monero did

* Replace RangeCommitment with Commitment

* Hard-code BP+ h to Ed25519's generator

* Use pub(crate) for BP+, not pub

* Replace initial_transcript with hash_plus

* Rename hash_plus to initial_transcript

* Finish integrating the FCMP BP+ impl

* Move BP+ folder

* Correct no-std support

* Rename "long_n" to eta

* Add note on non-prime order dfg points

coins/monero/src/ringct/bulletproofs/plus/transcript.rs

@@ -0,0 +1,24 @@
+use std_shims::{sync::OnceLock, vec::Vec};
+
+use dalek_ff_group::{Scalar, EdwardsPoint};
+
+use monero_generators::{hash_to_point as raw_hash_to_point};
+use crate::{hash, hash_to_scalar as dalek_hash};
+
+// Monero starts BP+ transcripts with the following constant.
+static TRANSCRIPT_CELL: OnceLock<[u8; 32]> = OnceLock::new();
+pub(crate) fn TRANSCRIPT() -> [u8; 32] {
+  // Why this uses a hash_to_point is completely unknown.
+  *TRANSCRIPT_CELL
+    .get_or_init(|| raw_hash_to_point(hash(b"bulletproof_plus_transcript")).compress().to_bytes())
+}
+
+pub(crate) fn hash_to_scalar(data: &[u8]) -> Scalar {
+  Scalar(dalek_hash(data))
+}
+
+pub(crate) fn initial_transcript(commitments: core::slice::Iter<'_, EdwardsPoint>) -> Scalar {
+  let commitments_hash =
+    hash_to_scalar(&commitments.flat_map(|V| V.compress().to_bytes()).collect::<Vec<_>>());
+  hash_to_scalar(&[TRANSCRIPT().as_ref(), &commitments_hash.to_bytes()].concat())
+}