Add aggregate expression

[wgtmac]

Add Aggregate to the Expression family. The current expression implementation looks pretty much similar to the Java implementation. The work items would include:

• Aggregate
• UnboundAggregate
• BoundAggregate
• CountAggregate: include CountNull,CountNotNull,CountStar
• ValueAggregate: include MaxAggregate and MinAggregate
• AggregateEvaluator

[SuKi2cn]

Hi, I’ve opened PR #335 implementing aggregate expressions and evaluator for this issue.
When you have time, I’d really appreciate any feedback on the design and implementation.