One gem to rule them all

[solnic]

This gigantic PR moves core, repository and changeset gems into main lib dir and its corresponding spec dir effectively turning 4 gems into just one.

This will make maintenance much simpler and will simplify managing releases too.

TODO

• [x] merge core
• [x] merge repository
• [x] merge changeset
• [x] simplify rake tasks
• [x] figure out why changeset timestamp plugin spec is failing randomly
• [x] ensure that merged docsites work on the rom-rb.org side
• [x] ~move and unify benchmarks~ meh, benchmarks need more work, this should be addressed in a separate PR