Check if data exists in collections

[xushiwei]

As #675, we have support selecting data from collections:

selected := {x.score for x <- a, x.name == "du"}
selected, ok := {x.score for x <- a, x.name == "du"}

Now we can support a special case of this form:

exists := {for x <- a, x.name == "du"}

That is, we don't select any data, but only check if data exists or not. For example:

type student struct {
	name  string
	score int
}

a := [student{"ken", 90}, student{"du", 84}, student{"wang", 70}]

duScore := {x.score for x <- a, x.name == "du"}
unknownScore, ok := {x.score for x <- a, x.name == "unknown"}

hasFailed := {for x <- a, x.score < 60}  // is any student failed?
hasFullMark := {for x <- a, x.score == 100} // is any student full mark?

[xushiwei]

https://github.com/goplus/gop/releases/tag/v0.9.4