Convert call and construct APIs to take VarScope scopes

[aardvark179]

This is the first two commits of #2330, and will be followed by PRs that convert the use sites to the new type. We can stage this because the when converting method references to functional interfaces the Java compiler allows for widening of types, in this case VarScope to Scriptable.

[aardvark179]

I think the recursive ConsString problem is likely an intermittent failure, and I'm not really sure what this test is trying to exercise. If we're interested in the total number of appends then we should simply increase the ConsString append limit, or change the strategy of what is being appended. At the moment the four thousand recursive iterations are only really testing that the stack per call is always below a limit along with the test harness parts.