Add ISO 8601 date parser

[pitdicker]

@djc #1143 seems a bit too large to review. But the first few commits make sense on their own.
This is the commit that adds an ISO 8601 date parser.

The parser would be used by NaiveDate::parse_from_iso8601().
It is a component for the ISO 8601 parsers for NaiveDateTime and DateTime<FixedOffset>.

But ISO 8601 also specifies that the date components of a duration can use the date formats supported by this parser, with the exception of the formats for a week date.

[codecov]

Codecov Report

Merging #1296 (37ba280) into 0.4.x (e730c6a) will increase coverage by 0.05%.
The diff coverage is 97.14%.

@@            Coverage Diff             @@
##            0.4.x    #1296      +/-   ##
==========================================
+ Coverage   91.35%   91.41%   +0.05%     
==========================================
  Files          38       39       +1     
  Lines       17034    17173     +139     
==========================================
+ Hits        15562    15698     +136     
- Misses       1472     1475       +3     

Files Changed	Coverage Δ
src/format/mod.rs	85.04% <ø> (ø)
src/format/parse_iso8601.rs	96.39% <96.39%> (ø)
src/format/parse.rs	97.30% <100.00%> (+0.06%)	⬆️
src/naive/date.rs	96.35% <100.00%> (+0.05%)	⬆️

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[pitdicker]

Maybe we should move all the ISO 8601 parsing code into a module of its own?

Edit: moved to a new module.

[pitdicker]

This enum serves no purpose yet. We need it to make sure that the date, time and offset components use the same format when parsing to a NaiveDateTime, DateTime or duration.

[pitdicker]

To reuse this method for parsing a duration it should take a flag to indicate it should not parse week date formats.

[djc]

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 😆.

As far as motivations go, I find these to be fairly weak, so I don't think I'm going to prioritize reviewing these for the near future.