Introduce the HasCompiler typeclass (fixes #4639)

[snoyberg]

There may be additional places where the typeclass needs to be added as
a constraint, I don't claim to have found all cases.

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!