Implement subtraction operator for GE class in BIP-0374 reference code

This commit implements the subtraction operator (sub) for the GE (Group Element) class in the secp256k1.py file as requested in the TODO comment in reference.py.

The implementation is straightforward, leveraging the existing neg method to define subtraction as addition with the negated element: self + (-a).

After implementing the operator, the code in reference.py was simplified by replacing expressions like:
s * G + (-e * A) with s * G - e * A

This makes the code more readable and directly matches the mathematical notation used in the BIP-0374 specification.

Co-Authored-By: Sebastian Falbesoner <sebastian.falbesoner@gmail.com>

bip-0374/reference.py

@@ -87,11 +87,10 @@ def dleq_verify_proof(
     s = int.from_bytes(proof[32:], "big")
     if s >= GE.ORDER:
         return False
-    # TODO: implement subtraction operator (__sub__) for GE class to simplify these terms
+    R1 = s * G - e * A
-    R1 = s * G + (-e * A)
     if R1.infinity:
         return False
-    R2 = s * B + (-e * C)
+    R2 = s * B - e * C
     if R2.infinity:
         return False
     if e != dleq_challenge(A, B, C, R1, R2, m, G):

bip-0374/secp256k1.py

@@ -240,6 +240,10 @@ class GE:
             return self
         return GE(self.x, -self.y)
 
+    def __sub__(self, a):
+        """Subtract a group element from another."""
+        return self + (-a)
+
     def to_bytes_compressed(self):
         """Convert a non-infinite group element to 33-byte compressed encoding."""
         assert not self.infinity