Myrial UDAs

This PR adds UDAs to Myrial.  JSON compilation is not implemented.  Also, there is no support for combine/merge.

One reasonable question is: should we replace all `BuiltinAggregateExpression`s by concrete `UdaAggregateExpression`s? FWICT there's not a strong reason not to. Only in the case of `COUNTALL` is the backend able to do anything unique or special. (You may have brought this up before).