Some projects need the connected component traversal feature.

However, that feature was removed from *ena* for performance reasons: preserving the data required for connected component traversal introduced an overhead that was unnecessary in the large majority of cases. (d0a8975)

This PR reintroduces this as an optional feature through a type parameter on the `UnificationStore`.

Because no extra data is stored when not using `ConnectedComponentTraversal`, this feature has no impact on performance when not used.

**Benchmarks:**
master:
```
test unify::tests::big_array_bench_InPlace             ... bench:     499,038 ns/iter (+/- 2,306)
test unify::tests::big_array_bench_clone_InPlace       ... bench:     614,115 ns/iter (+/- 2,672)
test unify::tests::big_array_bench_in_snapshot_InPlace ... bench:     741,977 ns/iter (+/- 24,403)
```
this branch:
```
test unify::tests::big_array_bench_InPlace             ... bench:     467,384 ns/iter (+/- 10,037)
test unify::tests::big_array_bench_clone_InPlace       ... bench:     571,047 ns/iter (+/- 3,150)
test unify::tests::big_array_bench_in_snapshot_InPlace ... bench:     740,714 ns/iter (+/- 7,666)
```

Compared to a feature, this design enables the library to be used simultaneously in both cases (connected component traversal required and not required), while benefiting from optimal performance in both cases.