print() andstr()` generics.These are contructed to be more easily extensible since you can override
the _header(), _data() or _footer() components individually. The
default methods are built on top of format().
vec_print(x, ...)vec_print_header(x, ...)
vec_print_data(x, ...)
vec_print_footer(x, ...)
vec_str(x, ...)
vec_str_header(x, ...)
vec_str_data(x, ...)
vec_str_footer(x, ...)
A vector