The public header files are moved to a new subdirectory, to separate
the definition of the public interface from the actual implementation.
Using an identical directory layout as the final installation has the
advantage that the example code can be build outside the project tree
without any modifications to the #include statements.
We accept NULL pointers if the size is zero, but the C standard
library functions memcpy and memmove requires valid pointers, even if
the size is zero.
