Introduce a Duration object

The main purpose of using a pull request for this change is to have one entry point for the discussions leading up to it: https://github.com/cucumber/cucumber/commit/c6535ce8dd20f43dbc9e26081a08f7d48afadcc6#commitcomment-7953669, and on the first commit on this branch, see below.

Detailed description:
Introduce a Duration object in Cucumber::Core::Test::Result

For results which do not have an actual duration (skipped, undefined and pending) instead hold an null object. Client can either use the "null duration" (0), or check if it is an actual duration or an null duration.
Agreed @tooky - #tap should yield the object itself. A good name for the method is either `#ms` or `#milliseconds` IMO - no need for underscores.

It seems to me that with this API, bug-free code would never call that method on the null object, so it doesn't really need to implement it. Or it could throw an error telling the client to use the `#tap` trick. I think returning self might be a bit surprising.