Added -s/--no-unused-secondaries option

`-s` removes warnings about unused varibles and values set together
with used ones, i.e. originating from the same last unpacking item
of rhs in assignment.

E.g. in

    local a, b, c = foo(), bar()
    return c

`-s` removes warning about unused `b` but not about unused `a`.

TODO: fix massive performance degradation.