Add PartialEq and Eq impls to the character case iterators

[Jules-Bertholet]

Allows easily checking whether a string is in a particular case.

Adds impl PartialEq<{ToUppercase, ToLowercase, ToTitlecase, char}> for {ToUppercase, ToLowercase, ToTitlecase} and impl Eq for {ToUppercase, ToLowercase, ToTitlecase}.

@rustbot label needs-fcp T-libs-api A-unicode

[rustbot]

r? @Mark-Simulacrum

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