Use "::" as separator for Fortran in cooked index

This teaches cooked_index_entry::full_name that "::" is the separator
for Fortran.  I don't know enough Fortran to write a test case for
this.  However, a different series I am working on has a regression if
this patch is not applied.

Approved-By: Simon Marchi <simon.marchi@efficios.com>