Potential bad performance due to use of Views

[lefou]

I don't use stringdiff myself, but came across the following PR in utest:

• Vendor stringdiff to fix pathological poor performance com-lihaoyi/utest#412

According to the PR description, there is some bad performance due to use of Views in the code base, which was fixed in the PR by patching a vendored version of stringdiff. Hopefully, it's possible to upstream the changes.

[yurique]

@lefou thank you for heads up!

as I mentioned in the comment in the linked PR - I did not know it was used in utest :), I'll apply the performance fixes here.