consistent file and class names for syntax.*Ops

* `ToXOps` and `XV` → `XOps` with file name `XSyntax.scala` or `XOps.scala`
  (latter if they don't contain `Syntax`)
* update Developer Guide
update `GenTypeClass`

* `Comonad` doesn't extend `Functor` directly
* take control over `IsEmpty`
* remove `ArrId`
* whitespace fixes
* move auxiliary functions into `////` blocks