Fix inner `final case class` warnings.
Removed an implicit.

Scala was warning about ambiguous implicits, even though `m` and `DimIdMonad` were actually the same value.
Make warnings fatal.
LogicalPlan.Let restricted to 1 binding.

Fixes #28.
Merge pull request #83 from sellout/28-single-binding-let

LogicalPlan.Let restricted to 1 binding.
Merge pull request #81 from sellout/minor-fixes

Fix inner `final case class` warnings.
Merge pull request #82 from sellout/13-mongodb-equi-joins

Some small changes along the way to mongo joins.
Fix an old two-arg let that slipped by.