src/include/clapp/parser.h: changed function parameter of basic_parser_t::reg from rvalue reference to value

src/clapp/value.cpp: replaced is_integral<>::value and is_[un]signed<>::value by is_integral_v<> and is_[un]signed_v<>

src/include/clapp/option.hpp: optimized gen_opt_conf_process_params() by forwarding param using std::forward()