Remove fold() closures from `FieldSet._hashCode`. (#554)

The fold() calls come with significant runtime cost, and make it more
difficult to hoist accesses to commonly used variables. Just rewriting
this pattern shows a significant performance improvement for both JS and
VM benchmarks.

Baseline

```
JS
HashCode(RunTime): 9833.333333333334 us.
HashCode(RunTime): 9891.625615763547 us.
HashCode(RunTime): 9607.655502392345 us.
HashCode(RunTime): 9661.835748792271 us.
HashCode(RunTime): 9765.853658536585 us.
VM
HashCode(RunTime): 4527.384615384615 us.
HashCode(RunTime): 4534.151583710407 us.
HashCode(RunTime): 4546.556818181818 us.
HashCode(RunTime): 4490.65470852017
```

Results

```
JS
HashCode(RunTime): 8004 us.
HashCode(RunTime): 7980.0796812749 us.
HashCode(RunTime): 7976.095617529881 us.
HashCode(RunTime): 7824.21875 us.
HashCode(RunTime): 7847.058823529412 us.
VM
HashCode(RunTime): 2474.5451174289246 us.
HashCode(RunTime): 2533.7037974683544 us.
HashCode(RunTime): 2532.4556962025317 us.
HashCode(RunTime): 2420.072551390568 us.
HashCode(RunTime): 2521.3198992443326
```

Co-authored-by: Loren Van Spronsen <lorenvs@google.com>