Add compression functions, features, docs, tests

[jasonmp85]

This adds a dependency on Hydra columnar and pg_cron.

Tests were all working prior to adding the columnar dependency. Getting it on the test machine may prove annoying.s

[ChuckHend]

Could it be installed using trunk? Here is an example of pg_cron being installed. Maybe that would work for columnar too?

[jasonmp85]

Could it be installed using trunk? Here is an example of pg_cron being installed. Maybe that would work for columnar too?

I avoided it previously because using the apt repositories was faster, but Hydra obscures where it gets its deb from this pgxman thing. I'll switch to trunk.

[jasonmp85]

Will add binary caching in a bit, but tests passing on all PG versions, even though adding Hydra makes local dev annoying (no Mac build), but will deal with that later.