internal/core/adt: make ListMarker Src an expression

This simplifies making a binary expression out of it. This
will help the new evaluator implementation.

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: Ia7a4698b28b658c577a67fab7ba303f2079bf089
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1172007
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mpvl

internal/core/adt/expr.go

@@ -367,7 +367,7 @@ func (x *Bytes) Kind() Kind       { return BytesKind }
 // vertices.
 
 type ListMarker struct {
-	Src    ast.Node
+	Src    ast.Expr
 	IsOpen bool
 }