Flexible offset parsing [2/3]

[pitdicker]

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The first commit is shared with https://github.com/chronotope/chrono/pull/1082. Then a couple of commits do some cleanup and add a new offset parser. All the test keep passing, except some of those for #z. The difference is in the error causes, which have improved a bit.

Next are commits to make %:z, %::z and %:::z work as documented:

• %:z should require a colon (this is what makes it different from %z).
• %::z should also be able to parse, and even require, seconds.
• %:::z should only accept hours.

Finally the FromStr implementation of DateTime<FixedOffset> now accepts optional seconds in the offset, so that it can parse the Debug format of DateTime.

---

TODO: I still need to figure out some way to test all the code paths of the new offset parser.

[pitdicker]

I did not realize %::z and %:::z are fairly recent additions, added in https://github.com/chronotope/chrono/pull/759. Looking at that PR I understand they never properly added to the parser. That seems like an extra argument to fix them.

I'll keep this PR as a draft for a little longer. https://github.com/chronotope/chrono/pull/1145 has the proper fix for some changes I had to make here to RFC 3339 parsing code, and it is better if that is merged first.

[codecov[bot]]

Codecov Report

Merging #1083 (7964d8f) into 0.4.x (00d389e) will increase coverage by 0.24%. Report is 6 commits behind head on 0.4.x. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            0.4.x    #1083      +/-   ##
==========================================
+ Coverage   86.10%   86.35%   +0.24%     
==========================================
  Files          37       37              
  Lines       13522    13776     +254     
==========================================
+ Hits        11643    11896     +253     
- Misses       1879     1880       +1     

Files Changed	Coverage Δ
src/format/mod.rs	75.36% <ø> (ø)
src/datetime/tests.rs	96.56% <100.00%> (+0.02%)	:arrow_up:
src/format/parse.rs	97.26% <100.00%> (+0.03%)	:arrow_up:
src/format/scan.rs	99.38% <100.00%> (+0.12%)	:arrow_up:
src/offset/fixed.rs	78.19% <100.00%> (+1.03%)	:arrow_up:

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