Use .hpp extension for private C++ headers

[ChrisThrasher]

All of the headers in src/ are actually C++ code since they use C++ headers and C++ symbols. We should use a .hpp extension for such files instead of .h.

[eXpl0it3r]

The title says it, I just want to reemphasize that this only applies to the private C++ headers, not the C headers in the include directory. 😄