Adds semigroup and monoid instances for Cofree

[mikesol]

Description of the change

The purpose of the pull request is to decrease the burden on library writers of creating newtypes for Cofree when they want semigroup and monoid instances. For example, in https://github.com/mikesol/purescript-wags-lib/blob/main/src/WAGS/Lib/Rate.purs and many other files in that project, custom Semigroup and Monoid instances are created using this pattern, which leads to lots of code duplication.

From my testing, it seems that these follow the Semigroup and Monoid laws.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)

[thomashoneyman]

This looks fine to me, but I'd like to leave this open for a little while so folks more familiar with Cofree (which I've rarely used) can weigh in. If no one else does, then I'm happy to merge in a week or so.

[garyb]

Seems sensible to me!