All builtins go in moorings.arr

Rationale --- there are way too many mutual dependencies between the built-ins,
e.g. they all want to use lists, and all want to use errors, and all want to
use equiv, etc.  So, we merge them all together into moorings.arr.

- Removed all uses of sugar like [] from moorings, to avoid confusing when
  testing it
- Consolidated all imports of libraries everywhere to use moorings.arr instead
  of all 5 imports
- Fixed minor bugs along the way in desugar-check.rkt and desugar.rkt
- Also added builtins.equiv and fixed desugaring of == to use that
ignore moorings