feat: ensure valid random PrivateKey if no secret is given

[charlie632]

fixes: #390

This will loop for an arbitrary time (10 times) until the random private key is valid.

The PrivateKeyImplementation can throw an error if an invalid secret is provided, which can happen if it returns a value bigger than 0xfffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141 (see https://crypto.stackexchange.com/questions/30269/are-all-possible-ec-private-keys-valid/30272#30272).

This looping mechanism follows a technique used by Apple

[what-the-diff]

PR Summary

• Enhanced Error Checking in Key Context Creation
  Added a verification step in secp256k1.swift to ensure the context for key creation is correctly initialized.

• Improved Private Key Generation
  Implemented a loop that tries to generate a random private key 10 times within the initialization function, enhancing the reliability of key generation process.

• Efficiency Improvement in Key Generation
  Refined the process of private key generation in the loop of the initialization function in secp256k1.swift for better efficiency.

• Verification of Public Key Serialization
  Included error checking after public key serialization operation to ensure its successful execution.

• Additional Check in Public Key Encoding
  Added a check following the serialization of the public key in the encode function to improve the trustworthiness of the coding process.

• Comprehensive Verification for Xonly Public Key Serialization
  Inserted a verification step after serializing the xonly public key in the publicKey computed property. This improves the security and correctness of the public key.

[csjones]

This is a great start @charlie632, thank you for the contribution! These changes just update the backing implementation, we still need to update the public API in Asymmetric.swift.

Additionally, we should incorporate a unit test (similar to the one discussed in the issue thread) with a high iteration rate. Maybe 100K?

Lastly, we'd want to update the examples in the README to reflect the API change.

[csjones]

We will want to update the function signature so a try catch statement isn't needed.

Suggested change

	@usableFromInline init(format: secp256k1.Format = .compressed) throws {
	@usableFromInline init(format: secp256k1.Format = .compressed) {

[csjones]

Hey @charlie632, just checking in, would you like any help with this PR?

[charlie632]

@csjones hey! Sorry, I've been extremely busy with a big deliverable for my company. I have some free time next week, so I can happily finish this PR!