Make isInfixOf tail-call optimized

[jfmengels]

• Added test to check that isInfixOf is not stack-safe. Without the change, the tests fails because of a stack overflow.
• Made isInfixOf tail-call optimized in all branches

Becnhmark: https://ellie-app.com/dnQMpprLrDPa1

Note: There are still quite a few functions that can be optimized in this package, but I likely won't propose PRs for these, as they become a bit more complex.

You can run the following to find them:

npx elm-review --template jfmengels/elm-review-performance/example --rules NoUnoptimizedRecursion

[Chadtech]

Awesome! Thanks again