Remove Monero torsion-free requirement and make output keys 32 bytes

Maintains the torsion-free requirement in the one place it's used (key 
images).

In the modern day, the output keys are checked to be points, yet in 
older protocol versions they were allowed to be arbitrary bytes.

Closes https://github.com/serai-dex/serai/issues/23 and 
https://github.com/serai-dex/serai/issues/25.

coins/monero/src/transaction.rs

@@ -2,7 +2,7 @@ use core::cmp::Ordering;
 
 use zeroize::Zeroize;
 
-use curve25519_dalek::edwards::EdwardsPoint;
+use curve25519_dalek::edwards::{EdwardsPoint, CompressedEdwardsY};
 
 use crate::{
   Protocol, hash,
@@ -46,7 +46,7 @@ impl Input {
       2 => Input::ToKey {
         amount: read_varint(r)?,
         key_offsets: read_vec(read_varint, r)?,
-        key_image: read_point(r)?,
+        key_image: read_torsion_free_point(r)?,
       },
       _ => Err(std::io::Error::new(
         std::io::ErrorKind::Other,
@@ -60,7 +60,7 @@ impl Input {
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct Output {
   pub amount: u64,
-  pub key: EdwardsPoint,
+  pub key: CompressedEdwardsY,
   pub view_tag: Option<u8>,
 }
 
@@ -72,7 +72,7 @@ impl Output {
   pub fn serialize<W: std::io::Write>(&self, w: &mut W) -> std::io::Result<()> {
     write_varint(&self.amount, w)?;
     w.write_all(&[2 + (if self.view_tag.is_some() { 1 } else { 0 })])?;
-    write_point(&self.key, w)?;
+    w.write_all(&self.key.to_bytes())?;
     if let Some(view_tag) = self.view_tag {
       w.write_all(&[view_tag])?;
     }
@@ -92,7 +92,7 @@ impl Output {
 
     Ok(Output {
       amount,
-      key: read_point(r)?,
+      key: CompressedEdwardsY(read_bytes(r)?),
       view_tag: if view_tag { Some(read_byte(r)?) } else { None },
     })
   }