Compute differences between DateIntervals with timeInterval(to:) and dateInterval(to:) methods.

[kielgillard]

Implements #326.

Please read #326 where I explain how I expect this needs a pitch and review and asks what I should do next.

What?

Returns the seconds between self and dateInterval or nil if there is no difference in time between them.
For example, given these two intervals on a timeline:

|-----|  <-- time interval --> |-----|

Returns a negative time interval when self ends before dateInterval starts. A postive time interval indicates self starts after dateInterval ends.
A return value of 0 indicates self starts or ends where dateInterval ends or starts (in other words, they intersect at their opposing start/end moments):

|-----|-----|

A return value of nil indicates self and dateInterval do not have any time between them:

|--|-----|--|

Testing

I have included unit tests.

I was not able to build and test on Xcode 15b3:

No such module 'FoundationMacros'

As such, I am relying on any CI to build and test these changes.

[MaxDesiatov]

@swift-ci test

[kielgillard]

I would have broken these down into separate test functions (one expecting non-nil and another nil intervals) but I implemented it like this to follow the lead of pre-existing test functions.

[iCharlesHu]

Hi @kielgillard! It looks like you are proposing a change to TimeInterval's public interface? Any changes to Foundation's API surface requires API Proposal: https://github.com/apple/swift-foundation/blob/main/Proposals/0000-template.md

Could you go ahead and draft up a proposal for this change so we can review them both here and on the forum? Thanks!

[kielgillard]

@iCharlesHu Hello, thank you for the comment. Would still love any feedback you might have!

I'm now working on a proposal in #331 and the implementation can now be found in #330.