Support an integer type annotation argument

[kanghyojun]

Support an integer type annotation argument which is one of the features that listed on #178. Since this is the blocker issue of #206, we can add constraints for integer types(int32, int64, bigint) after merging this PR.

However, I should create fixture.datetime.datetime-service, because all types except for a method of service couldn't compile the annotation. We have to compile the annotation of the unboxed type first to do #206.

Review, please :) @dahlia @Kroisse @AiOO.

[dahlia]

We could define a more general parser to take one or more digits and returns Integer, i.e., integer :: Parser Integer, and then make this annotationArgumentValue to utilize that.

[dahlia]

• Int has a limits on its size (up to 0x1FFFFFFF). We'd better to use Integer instead.
• Data constructors shouldn't be prefixed with A.

[checkmate-bot]

Checklist 🤔

src/Nirum/Parser.hs

• If a new reserved keyword is introduced, it has to be also added to reservedKeywords set in the Nirum.Constructs.Identifier module.

[dahlia]

Please use the past tense.

[codecov]

Codecov Report

Merging #267 into master will decrease coverage by 0.05%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master     #267      +/-   ##
==========================================
- Coverage   76.14%   76.08%   -0.06%     
==========================================
  Files          33       33              
  Lines        2452     2467      +15     
  Branches      131      132       +1     
==========================================
+ Hits         1867     1877      +10     
- Misses        454      458       +4     
- Partials      131      132       +1

Impacted Files	Coverage Δ
src/Nirum/Parser.hs	87.4% <100%> (+0.05%)	⬆️
src/Nirum/Constructs/Docs.hs	94.73% <100%> (ø)	⬆️
src/Nirum/Constructs/Annotation/Internal.hs	77.27% <50%> (-6.94%)	⬇️
src/Nirum/Targets/Python.hs	89% <66.66%> (-0.25%)	⬇️
src/Nirum/Constructs/Annotation.hs	91.89% <80%> (-2.4%)	⬇️

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