refactor: modularize elliptic curve implementation

Move the elliptic curve (EC) implementation into separate modules for
better code organization and maintainability. The `ec.rs` file was split
into `ec/mod.rs` and `ec/weierstrass.rs` to group the EC trait
definition and Weierstrass curve logic respectively. This change
improves modularity and clarifies the separation between different curve
types, making it easier to extend with additional curve types in the
future.

The core elliptic curve trait remains unchanged, while the Weierstrass
curve has been refactored into its own module. Unit tests have been
preserved and moved accordingly.

src/ec/mod.rs

@@ -0,0 +1,16 @@
+use crate::point::Point;
+use num_bigint::BigUint;
+
+pub mod weierstrass;
+pub use weierstrass::WeierstrassCurve;
+
+/// Base trait for all elliptic curves
+pub trait EllipticCurve {
+  fn add(&self, p1: &Point, p2: &Point) -> Point;
+  fn double(&self, p: &Point) -> Point;
+  fn mul(&self, p: &Point, scalar: &BigUint) -> Point;
+  fn is_on_curve(&self, p: &Point) -> bool;
+  fn order(&self) -> &BigUint;
+  fn base_point(&self) -> &Point;
+  fn field_modulus(&self) -> &BigUint;
+}