:nodoc: our SecureRandom patches [ci skip]

These methods shouldn't be added to `SecureRandom`, as they are neither secure nor random. The more appropriate place for this seems to be `Digest`, so we should move them there. (Pull request welcomed!)

Marking this `:nodoc:` for now, so we don't accidentally ship it as public API.

See https://github.com/rails/rails/pull/15306/files#r13055862 for details.