Update fourmolu version

[td202]

Purpose

Update the version of fourmolu used to format Haskell sources to 0.13.1.0.

Changes

• Update the CI.
• Update the readme.
• Update fourmolu.yaml to make (new) options explicit. Some hints and explicit fixities are given where fourmolu doesn't infer them.
• Haddock comments now begin with spaces.
• Singular constraints are formatted with parentheses.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[MilkywayPirate]

Looks great :)