Use memcpy() instead of bcopy()

memcpy() can be faster for non-overlapping memory regions.  Also using
constant length should be faster as optimizing compilers are expected
to fully inline and unroll the whole memcpy() call.
Reduce branching in Package-Merge

Add sentinel at the beginning of leaf_weight[] allows skipping bounds
check, which reduces branches in a tight loop.
Cache current leaf weight at current depth in curr_weight[]
Reorder code in package_merge()
Implement restartable collector

Now collector can potentially be stopped and restarted, allowing input
blocks to be much smaller than compression block.  This feature used
to be implemented in older versions of yambi.
Introduce order queue to compressor
Implement sequential collector
Avoid scanning in sequential mode