Changing header extension from .h to .hpp

The extension for C++ headers in Boost is .hpp, while .h indicates a C header.

In C8, the distinction is not important, since the code is C++-only, however, I use the popular Atom editor and it sometimes doesn't recognize a newly written header as C++. I thinks it is C and uses the wrong coloring and doesn't apply clang-format.

This could be easily fixed by renaming all headers to .hpp, since the extension is the most important guide for the editor.