Defaults or advice for choosing SignatureOptions

[mpartel]

It's not obvious how to choose good parameters for SignatureOptions. The docs do nicely explain what kind of effect the parameters have, but they don't make it easy to choose concrete values.

It might be good to

• impl Default for SignatureOptions, if universally reasonable defaults exist, or
• add a constructor that produces parameters that are good for e.g. a given file size and acceptable collision rate, or
• elaborate the documentation.