UInt256 type for Swift

[csjones]

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[what-the-diff]

PR Summary

• New Addition to secp256k1 directory
  We've included a newly developed file called UInt256.swift. This addition focuses on the enhancement and effective management of cryptographic operations.

• Updates to Asymmetric.swift
  A new initializer called init(_:format:) has been integrated to enable the creation of a secp256k1 private key from a raw representation. This provides another, arguably simplified, method for private key generation.

• Testing Updates in secp256k1Tests.swift
  We've introduced a new test case testUInt256() to validate the functioning of the newly added UInt256 type and private key initialization. This measure provides us with assurance of the efficiency of the new features.
  Further, we've upgraded the test case testPublicKeyWithCompressedPrivateKey() to take into account the effectiveness of the combined public key. This offers additional evidence of the synergistic working of our system components.