Proposal: Implement Tod::TimeOfDay#upto / Tod::TimeOfDay#step

Proposal for new instance methods `upto` and/or `step`

These can be used as following cases

```ruby
Tod::TimeOfDay('10:00').upto(Tod::TimeOfDay('11::00')) do |tod|
puts tod
end
#=> 10:00::00, 10:00:01, 10:00:02, ...10:59:59, 11:00:00
```

```ruby
Tod::TimeOfDay('10:00').step(Tod::TimeOfDay('11::00'), 60) do |tod|
puts tod
end
#=> 10:00, 10:01, 10:02, ...10:59, 11:00
```

c.f.
https://rubydoc.info/stdlib/date/Date#step-instance_method
https://rubydoc.info/stdlib/date/Date#upto-instance_method

before working on implementation, I would like to hear a comment about this methods. Thank you!

Start by reviewing the existing Tod::TimeOfDay API and the referenced Ruby Date#step and Date#upto documentation. Before coding, get agreement on the proposed method semantics, including the step argument and boundary behavior; done means those decisions are settled and the methods are implemented accordingly.

Written by the indexing model from the issue text.