Split function for anti-diff finger tree that is based on heuristics

[jorisdral]

Description

Closes #4268.

We add a splitSized function that computes a split and the corresponding split root measures using a heuristic, which the Sized class provides. One example of a sensible Sized instance would be one that computes the length of a finger tree. We can view the splitSized function as an equivalent function that delegates to splitl and splitr based on the heuristic.

We also perform comparative benchmarks, where we compare the performance of splitl, splitr and splitSized for three scenarios:

1. splitl's worst-case running time: a split at the end of the finger tree.
2. splitr's worst-case running time: a split at the start of the finger tree.
3. A split in the middle of the finger tree.

In scenario 1, splitl performs much better than splitr, which is expected, and splitc performs only slightly worse than splitl. In scenario 2, splitl performs much worse than splitr, which is expected, and splitSized performs only slightly worse than splitr. In Scenario 3, we see nearly identical performance for all three split functions.

Checklist

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[jasagredo]

Looks good to me. Thanks!