Add test case to Hamming that shows the difference between `len()` and `chars().count()` about rust HOT 5 CLOSED

In the case of hamming for DNA mutations the UTF-8 issue doesn't really come into play. Do we have any other exercises where we could expose this?

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In our current problem set? Not really. Anagrams can do it, but that's going to move much later in the sequence. Maybe in a problem we haven't implemented yet?

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Yeah, maybe, though there's nothing obvious in the list: http://exercism.io/languages/rust/contribute

Maybe we need a new exercise that focuses on this explicitly. Go has the same thing.

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Since #175 now deals with the difference between bytes and characters, I think the original intent behind this is in fact covered, even if the test was added to a different exercise than Hamming. What say you?

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Sounds good.

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