Consider sealing vector_math classes

[johnmccutchan]

@rakudrama @spydon Forking the conversation from the pull request.

[spydon]

Good idea with an issue!
I understand if you would want to mark the classes as sealed, then all of us that have extended any of the classes will of course have to create a new library that fits our needs better, but such is OSS life. :)

Like I said in the PR though, if you do mark them as sealed I hope that you also bump major, since it is a breaking change and will break all previous versions of Flame (and other apps and packages), since we have depended on that vector_math follows semver (which it has done so far) and as such we use the caret syntax.

And lastly; is there anything in my PRs blocking the classes from being sealed later, if not, why could they not be merged and released first? Since I doubt that vector_math will move over to sealed classes over night.