Avoid duplicate toCode calls in multiple Char.is... functions

[jfmengels]

This reduces the number of calls to toCode char. For instance, in the isAlphaNum function, the char argument would be converted to a char up to 3 times (once in isLower, isUpper and isDigit).

This makes it so that the conversion is done at most once, improving performance of the functions.

@lue-bird discovered this as an optimization for parsers (as part of stil4m/elm-syntax#237), where the Char.isAlphaNum function is used to identify variables.

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Functions like isUpper now make a function call to delegate to their dedicated helper. I don't know if it matters much (I assume a JIT compiler will optimize this) and we could duplicate the comparison logic in both isUpper and isUpperCode to avoid that, but I figured reducing duplication and therefore potential errors to be a reasonable default. I'mm happy to change this.

[github-actions]

Thanks for suggesting these code changes. To set expectations:

• Pull requests are reviewed in batches, so it can take some time to get a response.
• Smaller pull requests are easier to review. To fix nine typos, nine specific issues will always go faster than one big one. Learn why here.
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