Remove signed <-> unsigned warnings around size_t

[mrkline]

These get flagged by recent versions of GCC and Clang when building
with -Wsign-conversion.

[darealshinji]

Makes the build fail though on GCC 4.8.4:

In file included from <builddir>/main.cpp:27:0:
<builddir>/args.hxx: In member function ‘It args::ArgumentParser::Parse(It, It)’:
<builddir>/args.hxx:2722:72: error: expected type-specifier before ‘ptrdiff_t’
                                     const auto signedIdx = static_cast<ptrdiff_t>(idx);
                                                                        ^
<builddir>/args.hxx:2722:72: error: expected ‘>’ before ‘ptrdiff_t’
<builddir>/args.hxx:2722:72: error: expected ‘(’ before ‘ptrdiff_t’
<builddir>/args.hxx:2722:72: error: ‘ptrdiff_t’ was not declared in this scope
<builddir>/args.hxx:2722:72: note: suggested alternatives:
In file included from /usr/include/c++/4.8/utility:68:0,
                 from /usr/include/c++/4.8/algorithm:60,
                 from <builddir>/args.hxx:32,
                 from <builddir>/main.cpp:27:
/usr/include/x86_64-linux-gnu/c++/4.8/bits/c++config.h:187:28: note:   ‘std::ptrdiff_t’
   typedef __PTRDIFF_TYPE__ ptrdiff_t;
                            ^
/usr/include/x86_64-linux-gnu/c++/4.8/bits/c++config.h:187:28: note:   ‘std::ptrdiff_t’
In file included from <builddir>/main.cpp:27:0:
<builddir>/args.hxx:2722:87: error: expected ‘)’ before ‘;’ token
                                     const auto signedIdx = static_cast<ptrdiff_t>(idx);
                                                                                       ^

[Taywee]

I've pushed a change that uses an explicit cstddef and std::ptrdiff_t instead; that might fix that issue.

[darealshinji]

Now it works. Thanks.