Remove two booleans from map iterators

Remove handleZero/handleOne booleans from Iterators. Start iterating at -2, and use -2 and -1 as markers for zero and one sentinels.


Performance wise there seems to be no noticable changes in a quick test with 100 entries:

```
Benchmark                                                   Mode   Samples         Mean   Mean error    Units

Before change:
c.g.j.c.TestMapsBenchmark100.testIterate_GsIntObjectmap    thrpt        60     1017.885        1.211   ops/ms

After change
c.g.j.c.TestMapsBenchmark100.testIterate_GsIntObjectmap    thrpt        60     1013.230        3.875   ops/ms
c.g.j.c.TestMapsBenchmark100.testIterate_GsIntObjectmap    thrpt        60     1016.394        2.229   ops/ms
```