Add a uint8 "count" byte at the beginning of every row
Only build a table for benchmarking when we're running the benchmarks
Require that metric columns are non-null

It doesn't make much sense to have null metric columns. This makes the
collapsing of metric columns better-defined, and will allow us to remove
these nil checks on metric columns.
Collapse rows upon insert

A few notes:

* We're only using 1 byte to store the number of rows collapsed
  together, so we collapse only up to 256 of the same rows together in
  the same row slot. We could expand the counter to 2 bytes, but one
  benefit of using 1 byte is that it reduces the likelihood that the
  metric columns will overflow.
* Upon insert, we naively scan all rows in the segment and do byte
  comparisons. This may be prohibitively slow; we'll see.

This is currently very slow, and the benchmarks do not run because of
it. I'm in the process of profiling.