ISO 8601 parsers

[pitdicker]

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My main motivation for adding an ISO 8601 parser is that chrono is currently unable to parse is own Debug output if the year is outside the 0..=9999 range supported by RFC 3339. Edit: I forgot the FromString implementation of DateTime is very forgiving and accepts this.

A second motivation is that this crate frequently mentions ISO 8601. Then it is strange it does not even have a parser for that format :laughing:.

ISO 8601 date/time strings have a pretty complex format. I see it as a combination of 12 different formats for the date, 12 formats for the time, and 4 for the offset.

I have made the parser public as a parse_from_iso8601 method on NaiveDate, NaiveTime, NaiveDateTime and DateTime::<FixedOffset>.

Sorry that this is such a large PR. The only thing I can say is that about half of it are tests and documentation.

Fixes https://github.com/chronotope/chrono/issues/587.

[pitdicker]

I wondered if I could use floats as they may not be supported by all targets, and the CI gives the answer: no. I'll figure something out with just integers.

[pitdicker]

@jtmoon79 Thank you for the review. Sorry I am a bit slow slow with addressing your comments. I'll probably fix it up this week.

[codecov[bot]]

Codecov Report

Merging #1143 (afa45c6) into 0.4.x (e730c6a) will increase coverage by 0.18%. The diff coverage is 97.29%.

@@            Coverage Diff             @@
##            0.4.x    #1143      +/-   ##
==========================================
+ Coverage   91.35%   91.53%   +0.18%     
==========================================
  Files          38       39       +1     
  Lines       17034    17550     +516     
==========================================
+ Hits        15562    16065     +503     
- Misses       1472     1485      +13     

Files Changed	Coverage Δ
src/format/mod.rs	85.04% <ø> (ø)
src/format/parse_iso8601.rs	95.67% <95.67%> (ø)
src/datetime/mod.rs	86.96% <100.00%> (+0.87%)	:arrow_up:
src/datetime/tests.rs	96.59% <100.00%> (+0.09%)	:arrow_up:
src/format/parse.rs	97.30% <100.00%> (+0.06%)	:arrow_up:
src/format/scan.rs	99.26% <100.00%> (+0.01%)	:arrow_up:
src/naive/date.rs	96.35% <100.00%> (+0.05%)	:arrow_up:
src/naive/datetime/mod.rs	97.46% <100.00%> (+0.07%)	:arrow_up:
src/naive/datetime/tests.rs	98.71% <100.00%> (+0.08%)	:arrow_up:
src/naive/time/mod.rs	96.09% <100.00%> (+0.13%)	:arrow_up:

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