Improve performance initializing Mnemonics

[ccjernigan]

When Mnemonics is first touched, it performs an initialization of SecureRandom which is known to be slow.

While documenting this could be helpful, it doesn't give clients using this library a lot of options to work around it. That's because Mnemonics is an object, so initialization is effectively a JVM static initializer when means it runs as soon as something touches the object.

Possible alternative implementations:

• Defer SecureRandom initialization until it is needed, giving clients better control over timing
• Convert Mnemonics to a class, so that initialization happens during construction which allows clients better control over timing of the initialization
• Create a companion object initializer. This could be a suspend function, allowing secure random injection into a private constructor for Mnemonics
• ... There may be other options to consider as well