Add LuhnCheck function

[urbanfletch]

Add builtin in credit card validation function - LuhnCheckFunction

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[findepi]

@electrum are we ready to remove syntax-needs-review?

[electrum]

Please add documentation for this in presto-docs/src/main/sphinx/functions/string.rst

(String Functions doesn't seem like the right place to document this, but I can't find a better spot)

[urbanfletch]

Please add documentation for this in presto-docs/src/main/sphinx/functions/string.rst

(String Functions doesn't seem like the right place to document this, but I can't find a better spot)

We may ultimately want a validation functions library. There are a whole host of them you could write. But not worth it for just this one.

[electrum]

@martint thoughts on the name?

[martint]

I think luhn_check is fine. That's the name this algorithm is known by. Maybe luhn_validation, but either one works.

[electrum]

Looks good. Please rebase and I'll merge this.

[electrum]

Can you update the commit message to something like

Add luhn_check function

Easy way is to run git commit --amend then force push.

[electrum]

Thanks!