newtypes, Arbitrary instances, chores

[sorki]

• Couple of newtypes added replacing useless type aliases
• Two constructors hidden as we have smart constructors for these
• Dropping StorePathSet alias
• Remotes IO testsuite is now called remote-io and lives in tests-io dir to make space for "normal" testsuite
• Arbitrary instances moved to lib, so they can be used in remote as well
  • Some proved redundant and they are gone now along with tests/Arbitrary.hs
• Changelogs cleaned-up and renamed to be consistent