Fix diff failure due to Swift 5.6 Substring hashing change

[kmcbride]

Swift 5.6 appears to have a behavioral change (bug?) related to generating the hashValue for Substring instances. This causes dictionary lookups to miss and the resulting debug diff to contain incorrect data.

[codecov]

Codecov Report

Merging #188 (aea6b68) into master (6f7fec0) will not change coverage.
The diff coverage is 100.00%.

❗ Current head aea6b68 differs from pull request most recent head 95e66a8. Consider uploading reports for the commit 95e66a8 to get more accurate results

@@           Coverage Diff           @@
##           master     #188   +/-   ##
=======================================
  Coverage   95.37%   95.37%           
=======================================
  Files          42       42           
  Lines        1427     1427           
=======================================
  Hits         1361     1361           
  Misses         66       66           

Flag	Coverage Δ
macspm	95.37% <100.00%> (ø)

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Impacted Files	Coverage Δ
MobiusTest/Source/SimpleDiff.swift	100.00% <100.00%> (ø)

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