Use CaseInsensitive for domain part to follow RFC

[albertov]

According to RFC 1035, section 3.1:

Name servers and resolvers must compare [domains] in a case-insensitive manner

So I believe the Eq and Ord instances of EmailAddress should take it into account. This PR implements it in a API preserving way which didn't need tweaking any tests by using Data.CaseInsensitive.CI to store the domain part- A domainPartOriginal is provided to extract it in the original case in which it was parsed. Perhaps, fort better backwards compatibility, it should be the other way around? (ie: provide a domainPartFoldedCase to extract it folded and leave domainPart returning the original case)

[Porges]

I think it makes sense for the domain part but it’s going to be a breaking change to do this so would need to be in a major version update. The local part should not be case-insensitive as the interpretation is meant to be entirely up to the destination server (which could allow case-sensitive emails, although that would be unusual!)

[albertov]

Indeed! The local part change slipped in, it’s something we finally decided to do internally and wasn’t intended to be proposed upstream. I’ll clean that up and bump version tomorrow

[ddssff]

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