0.5: Revise exposed crate features

[pitdicker]

Remove the old crate features libc, old_time, rkyv and winapi.
Switch to the dep: syntax for optional dependencies that where exposed as features: pure-rust-locales, android-tzdata, iana-time-zone, js-sys, wasm-bindgen and windows-targets.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.99%. Comparing base (58ae538) to head (87f8b43).
Report is 1 commits behind head on 0.5.x.

Additional details and impacted files

@@           Coverage Diff           @@
##            0.5.x    #1552   +/-   ##
=======================================
  Coverage   93.99%   93.99%           
=======================================
  Files          37       37           
  Lines       16536    16543    +7     
=======================================
+ Hits        15543    15550    +7     
  Misses        993      993           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[pitdicker]

Also updated the description in lib.rs and the readme, and removed the __unstable_bench feature.

The bench feature was only used for benchmarking YearFlags::from_year. All that method does is:

let year = year.rem_euclid(400);
YEAR_TO_FLAGS[year as usize]

A division and a table lookup doesn't seem all that interesting to benchmark, let alone have a feature for.

[djc]

Much better, I think that split was definitely worth it!

[pitdicker]

Sorry that this is taking so many back-and-fort between us. But I'm really starting to like the changes here as good cleanups.

[djc]

So I think we should revert this change, because now without std is pretty much meaningless.

[pitdicker]

With the commit I added now disables no_std, with the goal that we only depend on the standard library if the implementation for that platform requires it.

So the features describe the functionality: now enables Utc::now, std enables the Error trait and TryFrom<SystemTime>.

Is that sensible or do you really want to revert?

[djc]

I think now should enable calling Utc::now() on most common platforms. If it doesn't, that feels broken.

[pitdicker]

It does. https://github.com/chronotope/chrono/pull/1552/files#diff-b1a35a68f14e696205874893c07fd24fdb88882b47c23cc0e0c80a30c7d53759R474

[djc]

It seems surprising that the cfg(feature = "std") guard doesn't match the behavior of whether we enable/disable no_std exactly (for the library).

[pitdicker]

It's a choice we can make. Separating the features allows things like using Utc::now in combination with wasmbind without depending on the standard library.

[djc]

Okay, let's try it. Easier to add it later than to remove it.