Do not install bits.h to the system.

After the removal of the filesystem memory access functions from
libleornian (starting at v0.2), the left overs (bits.h) have no use
outside the libleornian pattern analysis algorithm.
Create a skeleton header and .c.

This commit removes the single leo() placeholder interface function
prototype that would provide all libleornian access, and adds two
separate functions: leo_in() and leo_out().

These functions will be non-threaded implementations of input and output
processing (primarily pattern analysis). Both functions take a single
argument serving as a callback providing access to either the input our
output functions.

stddef.h has been included for size_t in the callback function argument
type declaration.
Make leo_in() and leo_out() accept userdata.

Previously both functions' callback prototypes had a void **userdata
argument, but the leo_in() and leo_out() functions themselves did not
accept userdata as a separate parameter to pass to their respective
callbacks.