Interposed functions should be internal not private

[cd1m0]

Given this sample:

contract Base {
   /// #if_succeeds true;
   function foo() internal returns (uint) {}
}

/// #invariant foo() == 1;
contract Child is Base {
}

The foo function in the base will be renamed to __original_Base_foo and made private.
However in the instrumentation of Child we will try to use the interposed function (__original_Base_foo) which is no longer accessible in Child since its now private.

To fix this we should make interposed functions internal.

[blitz-1306]

This is addressed by #258. Closing.