avoid mem leakage inside of sc_variable_vector resize

[hliu71]

here unique_ptr is used for values
std::vector<std::unique_ptr<sc_variable>> values;
std::function<sc_variable(char const, size_t)> creator;

I test the implementation in archsim, scenario is passing.

Limitation is
resize(10, 5);
resize(5,8);

values' size will be schrinked, but the value of elements are still 5
(similar to std::vector).
to remove limitation, updates required-:)
initial version to avoid mem leakage