Bug fixed: macros in the middle of a goto cursor or goto end-of-script were

Bug fixed: macros in the middle of a goto cursor or goto end-of-script were
not executed.

Explanation: the current goal was set only externally, but it is used by
macro disambiguation.

To fix the bug I have finally changed the type of a "lazy_macro" so that its
input is now a context. it used to be an integer, the selected goal; however,
the integer does not make sense since a macro can be invoked also when there is
no current proof.