Ruby: Context sensitive instance method resolution

[hvitved]

This PR implements call-sensitive instance method resolution, using the data-flow library's built-in support.

For example, in

class A
  def method1 x
    sink x
  end

  def method2 x
    method1 x
  end
end

class B < A
  def method1 x
    non_sink x
  end
end

B.new.method2(taint)

we will no longer report flow from taint to sink (but still to non_sink).

[aibaars]

Looks good to me.

[aibaars]

This predicate returns direct sub classes only, doesn't it? Let document that and perhaps also make it more clear from its name.

[aibaars]

Perhaps also add C === x tests. If I'm not mistaken, both types of case expressions are implemented using that operator.

[aibaars]

Ah I see you just moved these predicate. Perhaps we should post-pone === for a future PR. We might want to desugar case statements into if at some point too.

[hvitved]

Yes, we can do that in another PR. I guess is could be added to hasAdjacentTypeCheckedReads.