Lots of refactors and documentation

[jberryman]

• Remove Joinable class with single instance
• Refactor mkQuery and produceBatch for a little more clarity, so they only take and return relevant data.
• Simplify rebuildFieldStrippingRemoteRels; there seemed to be some unnecessary and also insufficient filtering logic happening here (but could use a close review)
• factor out the remote query aliasing/sorting stuff into enumerateRowAliases, unEnumerateRowAliases which we can document properly
• Remove some unnecessary fields from Batch; narrowing scope of functions that take or return a Batch aids understanding.
• inline stuff into extractRemoteRelArguments and refactor
• add coerceSet to Hasura.Prelude (this was used several times in a previous iteration, now only appears once; I'm not super committed to keeping it)
• add some tests, mostly just trying to excercise a few different things in the same query and used as an aid to understanding the code

[tirumaraiselvan]

I have kind of ignored 2 functions: insertBatchResults and extractRemoteRelArguments for now. Will get to it after the pending comments are resolved.

[jberryman]

Sorry I ran out of time/energy to think of a better name for RemoteRelBranch . What does the "Ann" in RemoteRelAnnField mean? For now I added a bit more explanation and a TODO

[tirumaraiselvan]

Sorry I ran out of time/energy to think of a better name for RemoteRelBranch . What does the "Ann" in RemoteRelAnnField mean? For now I added a bit more explanation and a TODO

Ann is just our internal abbreviation for annotation. It's fine. Let's do this later (if at all).