GitLab

(There might be a cleverer way to do this, but it's beyond me at this
present moment, so smashing it is.  And I'll give up my
"no abstractions in benchmarks mantra" for this one enough to put a
value table together and pay the cost to offset into it.)

Confirmed, things do get better at larger scale.

```
pkg: github.com/ipld/go-ipld-prime/_rsrch/nodesolution/impls
BenchmarkMap3nBaselineNativeMapAssignSimpleKeys-8                6062440               199 ns/op             256 B/op          2 allocs/op
BenchmarkMap3nBaselineJsonUnmarshalMapSimpleKeys-8                520588              2308 ns/op             672 B/op         18 allocs/op
BenchmarkMap3nFeedGenericMapSimpleKeys-8                         2062002               626 ns/op             520 B/op          8 allocs/op
BenchmarkMap3nFeedGennedMapSimpleKeys-8                          2456760               489 ns/op             416 B/op          5 allocs/op
BenchmarkMap3nFeedGennedMapSimpleKeysDirectly-8                  2482074               468 ns/op             416 B/op          5 allocs/op
BenchmarkMap3nBaselineNativeMapIterationSimpleKeys-8            15704199                76.0 ns/op             0 B/op          0 allocs/op
BenchmarkMap3nGenericMapIterationSimpleKeys-8                   19439997                63.0 ns/op            16 B/op          1 allocs/op
BenchmarkMap3nGennedMapIterationSimpleKeys-8                    20279289                59.0 ns/op            16 B/op          1 allocs/op
BenchmarkMap25nBaselineNativeMapAssignSimpleKeys-8                726440              1457 ns/op            1068 B/op          2 allocs/op
BenchmarkMap25nFeedGenericMapSimpleKeys-8                         304988              3961 ns/op            2532 B/op         30 allocs/op
BenchmarkMap25nFeedGennedMapSimpleKeys-8                          388693              3003 ns/op            1788 B/op          5 allocs/op
BenchmarkMap25nFeedGennedMapSimpleKeysDirectly-8                  429612              2757 ns/op            1788 B/op          5 allocs/op
BenchmarkMap25nBaselineNativeMapIterationSimpleKeys-8            3132525               417 ns/op               0 B/op          0 allocs/op
BenchmarkMap25nGenericMapIterationSimpleKeys-8                   4186132               286 ns/op              16 B/op          1 allocs/op
BenchmarkMap25nGennedMapIterationSimpleKeys-8                    4406563               271 ns/op              16 B/op          1 allocs/op

pkg: github.com/ipld/go-ipld-prime/impl/free
BenchmarkMap3nFeedGenericMapSimpleKeys-8                 1177724              1026 ns/op            1216 B/op         13 allocs/op
BenchmarkMap3nGenericMapIterationSimpleKeys-8            3497580               344 ns/op             464 B/op          4 allocs/op
BenchmarkMap25nFeedGenericMapSimpleKeys-8                 156534              8159 ns/op            7608 B/op         62 allocs/op
BenchmarkMap25nGenericMapIterationSimpleKeys-8            393928              2543 ns/op            3632 B/op         26 allocs/op
```

Basically:

- the build time ratio of our maps to native maps actually gets better
  (I didn't expect this) (though native maps still win handily; which,
  still, is no surprise, since ours Do More and have to pay at least
  Some abstraction cost for all the interface stuff).

- the iterate time ratio of our maps to native maps *also* gets better;
  it's almost a full third faster.

- we can confirm that the allocations are completely amortized for our
  codegen'd maps (the count doesn't rise with scale *at all*).  Nice.

- our maps are admittedly still about twice the size in memory as
  a golang native map would be.  But this is no surprise with this
  current internal architecture.  And one could make other ones.

- and we can see the old design just out-of-control *sucking* at scale.
  Building still taking twice as long in the old design; and
  iterating taking -- yep -- 10 times as long.

I'm not sure if these tests will be worth keeping around, because it's
kinda just showing of some unsurprising stuff, but, eh.  It's nice to
have the expected results confirmed at a another scale.