crypto/cipher: add one-time pad encrypt and decrypt

[vivaidris]

Adds a One-Time Pad (OTP) implementation to crypto/cipher.

OTP is a type of encryption that is provably secure
information-theoretically when used correctly. It
requires a key that is the same length as the message
and used only once.

This package provides the following:

• OTPEncrypt and OTPDecrypt functions for encryption
  and decryption of byte slices.
• KeyGen function for generating cryptographically
  secure random keys of the appropriate length.
• A simple unit test (otp_test.go) that verifies
  encryption and decryption works correctly.

Note: OTP is not intended to be used in real-life practice
as it has highly limited capabilities and can only be used once
for the safest encryption, making it highly inefficient.

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