evaluate: set evaluation context for set elements

evaluate: set evaluation context for set elements

This resolves same issue as previous patch when such
expression is used as a set key:

        set z {
                typeof ct zone
-               elements = { 1, 512, 768, 1024, 1280, 1536 }
+               elements = { 1, 2, 3, 4, 5, 6 }
        }

Signed-off-by: Florian Westphal <fw@strlen.de>