Simplify sparsetable to use the new common serialization infrastructure written when adding densehashtable serialization.
Simplify sparsetable to use the new common serialization
infrastructure written when adding densehashtable serialization.

This should not (hopefully!) be user-visible; it's just cleaning up
the code somewhat, getting rid of some ugly macros, etc.

LeslieW/sparsehash,LeslieW/sparsehash,LeslieW/sparsehash