Add dict.each

[mooreryan]

Addresses #580.

• Adds dict.each implemented using dict.fold.
• Parameter names are modeled after those in the dict module.
  • The list.each function uses f rather than fun for the name of the iterating function, however.
• Tests are modeled after those for list.each.
• Docs are modeled after those for list.each.

[lpil]

Nice! Thank you. Would you mind updating the changelog also? 🙏

[mooreryan]

Thanks, I will update the changelog as well.

[mooreryan]

Ah shoot, I messed up the the git history here. Let me just close this and fix it.