1) TODO: I/O implementation in densehashtable.h

2) TODO: support non-plain-old-data types for key/value.  Ideally,
   identify POD situations and use malloc+memmove for them, otherwise
   use new+copying.

3) TODO: document SPARSEHASH_STAT_UPDATE macro, and also macros that
   tweak performance.  Perhaps add support to these to the API?

4) TODO: support exceptions?

5) BUG: sparsetable's operator[] doesn't work well with printf: you
   need to explicitly cast the result to value_type to print it.  (It
   works fine with streams.)

6) TODO: consider rewriting dense_hash_map to use a 'groups' scheme,
   like sparsetable, but without the sparse-allocation within a
   group.  This makes resizing have better memory-use properties.  The
   downside is that probes across groups might take longer since
   groups are not contiguous in memory.  Making groups the same size
   as a cache-line, and ensuring they're loaded on cache-line
   boundaries, might help.  Needs careful testing to make sure it
   doesn't hurt performance.

7) TODO: Get the C-only version of sparsehash in experimental/ ready
   for prime-time.

8) TODO: solve the problem that config.h pollutes the global namespace
   with all the #defines it makes (any number of which might conflict
   with a #define in another config.h a user might happen to include).

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20 March 2005