Make reaper workloads modifiable

[FinleyMcIlwaine]

It is convenient to be able to modify reaper workloads outside of the reaperAction in scenarios where we don't want to wait for reaperDelay for the jobs to be updated. This PR adds an atomic reaperModify function to the reaper API which enables this.

Since this changes the visible Reaper constructor, I did a major version bump (auto-update 0.1.6 -> 0.2.0) and added a changelog entry. I also bumped the warp dependency on auto-update upper bound.

Before submitting your PR, check that you've:

• [x] Bumped the version number
• [x] Documented new APIs with Haddock markup
• [x] Added @since declarations to the Haddock

After submitting your PR:

• [x] Update the Changelog.md file with a link to your PR

[kazu-yamamoto]

@snoyberg I'm OK with this PR. But I don't remember why the Reaper constructor is exported. Should we stop exporting it and export it from Reaper.Internal?

[kazu-yamamoto]

Merged anyway. I will probably make Reaper.Internal later. Thank you for your contribution!