Add DateTime::<Utc>::from_timestamp

[demurgos]

This commit adds the new constructor from_timestamp to build a DateTime<Utc> from a UNIX timestamp.

Figuring out how to convert a timestamp into a DateTime<Utc> was a common issue:

• Easy way to convert timestamp to UTC object? #88
• Converting a timestamp to local time is verbose and unintuitive #200
• Utc.timestamp method is unintuitive #832

This commit should make DateTime<Utc> creation more discoverable and intuitive.

This commit respects the current convention of preferring fallible functions. It avoids however the _opt suffix as there is no panicking variant. See this issue for discussion about error handling and panics.

• Supersedes Add DateTime::<Utc>::from_timestamp_opt #1278
• Closes Utc.timestamp method is unintuitive #832

[codecov]

Codecov Report

Merging #1279 (449d907) into 0.4.x (2960f2c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##            0.4.x    #1279   +/-   ##
=======================================
  Coverage   91.40%   91.41%           
=======================================
  Files          38       38           
  Lines       16932    16950   +18     
=======================================
+ Hits        15477    15495   +18     
  Misses       1455     1455           

Files Changed	Coverage Δ
src/datetime/mod.rs	86.61% <100.00%> (+0.38%)	⬆️
src/lib.rs	95.58% <100.00%> (ø)

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[djc]

Thanks!