Add 'isStringLike' attribute to Domain
Use same Operator type for relational and sql
Add toString method to Environment
Fix infinite loop (typo)
Fix isAggregate property of case branch
Fix expected type for branches
Move Operator.Typechecker to own module DomainChecker
Fix error message
RangeType -> RangeDomain
Replace schema(checkDomain:Boolean) with more specific methods
Merge relational's AggregationOp with Operator.

Other (string-based) Aggregator can be included by inheriting from
AggregationOperator.
Add Schema.toString
Don't seal class Operator

Allow to define other operators later on.
Fix domain of subset query expression
Add tests for relational algebra