The format specifier of std::sprintf is incorrect

[lm-kasuga]

The variable len is int, but %u is used.
This may cause UB(but the compiler doesn't say any errors or warnings. ref. https://stackoverflow.com/questions/11303784/why-doesnt-gcc-wformat-warn-about-printf-d-on-an-unsigned-int)
We should use %d instead, or change the type of len to unsigned int.
Using other C++ STL (like std::stringstream) is also good, I think.

blueoil/blueoil/converter/templates/include/npy.hpp

Line 104 in 3510e3f

std::sprintf(buf, "%c%c%u", c_endian, c_type, len);