more consisent interface
check if Sizeable is empty
added c_str() method

it’s handy anyway
use String rather than std::string
test empty method
use String rather than std::string
use String rather than std::string
Merge branch 'string_interface'

use String rather than std::string
There are still places where std::string is used, like eg. str() and repr()
and in the Error class. But OK