Fix two warnings about implicit casts

[alt-graph]

This PR fixes two warnings in bit_manip.h that appear when compiling with -Wconversion.

[Finii]

Hmm, did anyone see these warnings?

In file included from ../include/gul14/string_util.h:30:
../include/gul14/string_view.h:328:47: warning: operand of ? changes signedness: 'typename iterator_traits<const char *>::difference_type' (aka 'long') to 'unsigned long' [-Wsign-conversion]
        return iter == this->cend () ? npos : std::distance ( this->cbegin (), iter );
        ~~~~~~                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/trim.cc:36:27: note: in instantiation of member function 'gul14::basic_string_view<char>::find_first_not_of' requested here
    const auto lpos = str.find_first_not_of(ws_chars);
                          ^
In file included from ../src/trim.cc:24:
In file included from ../include/gul14/trim.h:28:
In file included from ../include/gul14/string_util.h:30:
../include/gul14/string_view.h:344:72: warning: implicit conversion changes signedness: 'gul14::basic_string_view::size_type' (aka 'unsigned long') to 'std::reverse_iterator<const char *>::difference_type' (aka 'long') [-Wsign-conversion]
        const_reverse_iterator iter = find_not_of ( this->crbegin () + pos, this->crend (), s );
                                                                     ~ ^~~
../src/trim.cc:41:27: note: in instantiation of member function 'gul14::basic_string_view<char>::find_last_not_of' requested here
    const auto rpos = str.find_last_not_of(ws_chars);
                          ^
In file included from ../src/trim.cc:24:
In file included from ../include/gul14/trim.h:28:
In file included from ../include/gul14/string_util.h:30:
../include/gul14/string_view.h:358:27: warning: implicit conversion changes signedness: 'typename iterator_traits<reverse_iterator<const char *>>::difference_type' (aka 'long') to 'unsigned long' [-Wsign-conversion]
        return len_ - 1 - std::distance ( first, last );
                        ~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../include/gul14/string_view.h:345:48: note: in instantiation of function template specialization 'gul14::basic_string_view<char>::reverse_distance<std::reverse_iterator<const char *>>' requested here
        return iter == this->crend () ? npos : reverse_distance ( this->crbegin (), iter );
                                               ^
3 warnings generated.

[Finii]

https://github.com/gul-cpp/gul14/actions/runs/4230859256/jobs/7348684691 Expand Build and run ASAN tests

Maybe we should -Werror?

[alt-graph]

Hmm, did anyone see these warnings?

... in any case there are more warnings, I just fixed them in a single source file.

[alt-graph]

Maybe we should -Werror?

I am not sure. This might also break the CI if Github decides to upgrade their compilers or whatever. Sure, it might uncover some interesting warnings, but is that worth it?

[soerengrunewald]

Maybe we should -Werror?

I am not sure. This might also break the CI if Github decides to upgrade their compilers or whatever. Sure, it might uncover some interesting warnings, but is that worth it?

If it breaks, we can fix it.