Added slabClassIndex to chunk.

This is meant to help future chunk-linking functionality, especially
to link chunks from different slab classes.
Added Arena.chunkMem() method.
Split out chunk addRef/decRef() helper methods.
Arena.chunk() also returns slabClass.
Support chainability of []byte buf's.

There was an "extra" next pointer slot used for tracking free-lists.
The chaining feature exposes that for application usage, allowing apps
to chain buffers using Arena.SetNext(buf, bufNext) and to navigate
chains using bufNext = Arena.GetNext(buf).

Some apps may use that to cut large byte memory arrays into many
smaller, more uniform-sized buf's and chain the buf's together, to
reduce fragmentation.

The chain tracks ref-counts, so the last DecRef() on the buf at the
head of a chain might cascade DecRef()'s down the chain.
Discuss chainability in README.