Use "key" instead of "value" in HashMap exercise

[PaulXiCao]

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[djmitche]

What motivates this change? The objective is to count values, so it seems like 'value' is a good name.

It uses a std::collections::HashMap to keep track of which values have been seen and how many times each one has appeared.

[PaulXiCao]

It is just that key/value have usually predefined meanings for hashmaps (associative containers in general). And as this is an exercise to for the API of hashmaps it might put some people off that the variable 'value' is used as a key.

Unsure if the proposed renaming is the best though. What about "values -> itemOccurences" and "value -> item"?

(Sorry about the formatting. I am on my phone atm.)

[djmitche]

I think a PR to change both the word used in the exercise and the variable name would be OK. itemOccurrences sounds pretty long, but item and items is probably OK!