Method capacity

tests/framework/src/doctest.cpp:655–661

    return data.size;
}
DOCTEST_GCC_SUPPRESS_WARNING_POP

String::size_type String::capacity() const {
    if(isOnStack())
        return len;
    return data.capacity;
}

String String::substr(size_type pos, size_type cnt) && {
    cnt = std::min(cnt, size() - 1 - pos);