Add secp256k1 implementation with core components, context management, and testing suite

This commit introduces a complete Go implementation of the secp256k1 elliptic curve cryptographic primitives, including field and scalar arithmetic, group operations, context management for enhanced security, and a comprehensive testing suite. Key features include optimized scalar multiplication, context creation and destruction, and basic ECDSA operations. The README has been updated to reflect the new structure and usage examples.

p256k1/hash.go

@@ -2,6 +2,7 @@ package p256k1
 
 import (
 	"hash"
+
 	"github.com/minio/sha256-simd"
 )