Patch and Tidy

• Corrects usage of Secure random number generator
• Change UTF8 handling to safer default (throwOnInvalidBytes: true)
  • .NET Encoding.UTF8 encoding instance does not raise exceptions used to encode bytes which cannot represent a valid encoding & will return the same 'unknown' character instead. This can cause entropy loss when converting from bytes to strings.
• Change secure equals to match .net identity implementation
• Inline vars in encipher method