Add numeric-constraints annotation

[earlbread]

Close #206

[codecov]

Codecov Report

Merging #271 into master will increase coverage by 0.37%.
The diff coverage is 58.82%.

@@            Coverage Diff             @@
##           master     #271      +/-   ##
==========================================
+ Coverage   76.11%   76.48%   +0.37%     
==========================================
  Files          34       33       -1     
  Lines        2906     2905       -1     
  Branches      195      198       +3     
==========================================
+ Hits         2212     2222      +10     
+ Misses        499      485      -14     
- Partials      195      198       +3

Impacted Files	Coverage Δ
src/Nirum/Targets/Python.hs	87.16% <58.82%> (-1.02%)	⬇️
..._64-osx/Cabal-2.2.0.1/build/autogen/Paths_nirum.hs
src/Nirum/Targets/Python/CodeGen.hs	88.53% <0%> (+0.63%)	⬆️

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[dahlia]

• Please write docs about the added @numeric-constraints annotation in the Common annotations section of the docs/annotation.md file.
• Could you rebase 7f549ca on master rather than merging master (i.e., 9d15860)?

[earlbread]

I wrote docs and rebased the branch. @dahlia

[dahlia]

Merged: f2edb0f.