support GroupBy clause using Grouping columns

[t2y]

I implemented GROUP BY clause referring ORDER BY clause.

To be useful with GroupBy, I think we need to give the built in function in sql with columns. genmai already expose Function struct, so I added Grouping struct to be able to use it with grouping columns.

I'm not sure genmai design, I need review and help.

• should I add general AS clause?

I added the feature to Function struct to be able to rename column name for minimum impacts. If genmai should support general AS clause, I will add the feature before GroupBy. What do you think?

type Function struct {
	...

	// alias name (optional).
	As string
}

• which is the priority number appropriate?

I just set 300 as same as OrderBy.

	return c.appendQuery(300, GroupBy, groupbys)

[t2y]

hmm, postgres has failed ...