Merge cryper.h into cses_internal.h
Include the concrete implementation name in the secretbox functions

(libsodium doesn't have the #defines the the _detached versions of its secretbox functions)
Use libsodium's memzero instead of our own implementation.

Also export it, since this is something a user would reasonably want to use since
they will be handling the server's secret at some point.
Change cses_crypto prefix to libcses