Basic sign conversion

[Brett Hale <brettyhale@…>]

Mature libraries like boost.filesystem shouldn't be giving these sorts of errors with '-Wconversion' checks...

/Users/brett/local/include/boost/filesystem/string_file.hpp:27:27: warning: 
      implicit conversion changes signedness: 'size_type' (aka 'unsigned long')
      to 'streamsize' (aka 'long') [-Wsign-conversion]
  file.write(str.c_str(), str.size());
  ~~~~                    ^~~~~~~~~~
/Users/brett/local/include/boost/filesystem/string_file.hpp:38:22: warning: 
      implicit conversion changes signedness: 'std::size_t'
      (aka 'unsigned long') to 'streamsize' (aka 'long') [-Wsign-conversion]
  file.read(&str[0], sz);