std: fix compile errors in std.crypto.ecc (ziglang#23797)

Implemented `neg()` method for `AffineCoordinates` struct of p256,
p384 and secp256k1 curves.

Resolves: ziglang#20505 (partially)

Author: HydroH

lib/std/crypto/pcurves/p256.zig

@@ -471,6 +471,10 @@ pub const AffineCoordinates = struct {
     /// Identity element in affine coordinates.
     pub const identityElement = AffineCoordinates{ .x = P256.identityElement.x, .y = P256.identityElement.y };
 
+    pub fn neg(p: AffineCoordinates) AffineCoordinates {
+        return .{ .x = p.x, .y = p.y.neg() };
+    }
+
     fn cMov(p: *AffineCoordinates, a: AffineCoordinates, c: u1) void {
         p.x.cMov(a.x, c);
         p.y.cMov(a.y, c);

lib/std/crypto/pcurves/p384.zig

@@ -471,6 +471,10 @@ pub const AffineCoordinates = struct {
     /// Identity element in affine coordinates.
     pub const identityElement = AffineCoordinates{ .x = P384.identityElement.x, .y = P384.identityElement.y };
 
+    pub fn neg(p: AffineCoordinates) AffineCoordinates {
+        return .{ .x = p.x, .y = p.y.neg() };
+    }
+
     fn cMov(p: *AffineCoordinates, a: AffineCoordinates, c: u1) void {
         p.x.cMov(a.x, c);
         p.y.cMov(a.y, c);

lib/std/crypto/pcurves/secp256k1.zig

@@ -549,6 +549,10 @@ pub const AffineCoordinates = struct {
     /// Identity element in affine coordinates.
     pub const identityElement = AffineCoordinates{ .x = Secp256k1.identityElement.x, .y = Secp256k1.identityElement.y };
 
+    pub fn neg(p: AffineCoordinates) AffineCoordinates {
+        return .{ .x = p.x, .y = p.y.neg() };
+    }
+
     fn cMov(p: *AffineCoordinates, a: AffineCoordinates, c: u1) void {
         p.x.cMov(a.x, c);
         p.y.cMov(a.y, c);