Make RegexFlags a newtype and add a Newtype instance

[triallax]

Description of the change

Make RegexFlags a newtype and add a Newtype instance for it. I just thought that since it's merely wrapping a record, making it a newtype sounds appropriate.

I also refactored the Eq instance.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)

[garyb]

Maybe should be listed under breaking changes? I don't know whether we count breaking FFI representation as a breaking change, as I'm pretty sure we encourage people to generally rely on a particularly representation for a type (unless it's known to be a newtype, ironically).

[JordanMartinez]

Maybe should be listed under breaking changes? I don't know whether we count breaking FFI representation as a breaking change, as I'm pretty sure we encourage people to generally rely on a particularly representation for a type (unless it's known to be a newtype, ironically).

As far as I know, we don't. And if they do, then they implicitly agreed to deal with such problems if/when they ever arise, right?

[MonoidMusician]

How about adding a Newtype instance for it too?

[triallax]

@MonoidMusician I'm fine with that, but does it bring any real benefits?

[MonoidMusician]

Having a newtype instance does make some manipulations easier. But I'm asking for it mostly for consistency with the other newtypes in this library (e.g. Pattern and Replacement) and because it should be there if it can exist so that it doesn't have to be added in a follow-up PR once someone does need/want it down the road.