use *.hpp extension

we've got 2 different extensions for header files, so we should choose one.

i used to work w/ a good c++ programmer, and he said it was better to use header extensions that differentiate between c and c++ sources. that's literally the only shred of anecdotal evidence i can use to argue for `*.hpp`.

@calvinmm you cool with moving `hmm.h` &rarr; `hmm.hpp`?