Include default flag values in --help

[snoyberg]

Fixes #893. The implementation strategy is to replace First Bool with
either a First True or First False so that we can use the type to
ensure we display the correct default. This results in a lot of changes
to the codebase, but they are all type directed.

Alternatively, we could introduce a typeclass here to reduce some
boilerplate, but I prefer the more explicit approach.

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

Please include the following checklist in your PR:

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!