Rename Duration to TimeDelta, add type alias

[pitdicker]

We have discussed something like this many times. Reasons to rename Duration to TimeDelta:

• A Duration is usually understood to be strictly positive. Our type can be both positive and negative. That makes it more flexible, which is a good thing for a date and time library. The name TimeDelta can better represent a signed value.
• The standard library made its Duration type unsigned, as that is a better match for system APIs. We want to avoid confusion with that type.
• The name OldDuration we currently use as an alias in our documentation to ease the confusion a little is just sad (and not really helping).
• On our 0.5 branch we have already renamed it to TimeDelta, and we expect users to eventually migrate. By making the name available now we can ease the migration a little when the time comes.
• The name difference between our 0.4 and 0.5 branches is my biggest source of merge conflicts. It would make me happy if we can make them the same.

The old name Duration will remain available as a type alias for TimeDelta, so this is not a breaking change.

[codecov]

Codecov Report

Attention: 41 lines in your changes are missing coverage. Please review.

Comparison is base (f4b4701) 91.84% compared to head (4be71c2) 91.85%.

Files	Patch %	Lines
src/time_delta.rs	95.85%	31 Missing ⚠️
src/datetime/mod.rs	78.26%	5 Missing ⚠️
src/naive/datetime/mod.rs	96.80%	3 Missing ⚠️
src/date.rs	92.59%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1406   +/-   ##
=======================================
  Coverage   91.84%   91.85%           
=======================================
  Files          38       38           
  Lines       17518    17526    +8     
=======================================
+ Hits        16090    16098    +8     
  Misses       1428     1428           

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[pitdicker]

Did a review of all lines. It was not a trivial find-and-replace, but it seems like I didn't mangle things.

[djc]

You refer to TimeSpan a number of times in your OP. Which is it -- TimeDelta or TimeSpan?

[pitdicker]

You refer to TimeSpan a number of times in your OP. Which is it -- TimeDelta or TimeSpan?

TimeDelta. Working on chrono makes me feel dumb surprisingly often.

[pitdicker]

There was a little discussion on possible names in #1137 (comment). I don't care deeply about the names, but TimeDelta and TimeSpan have my preference. It seems you and @esheppa had a preference for TimeDelta as that is what is already on the 0.5 branch.

[djc]

Okay -- do we want to deprecate the Duration alias, and if so, when?

[djc]

This should be formatted as a type, that is, with empty lines around it and not between a bunch of imports.

[djc]

Let's use "Time delta" in the title here?

[pitdicker]

Thank you! Looking forward to less merge conflicts (going to introduce plenty of new ones though now that I'm starting to work on the 0.5 branch).

Okay -- do we want to deprecate the Duration alias, and if so, when?

I feel that deprecating the type would cause a lot of churn while bringing little benefit. New code can start using TimeDelta and be compatible with 0.5. But personally I'd leave it available until 0.5.