Implement operators as functions

[MidnightDesign]

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[rieschl]

lots of bc breaks in there?

[MidnightDesign]

lots of bc breaks in there?

I was very deliberate when deciding what's @api and what's @internal originally. All of the removed classes were @internal ... except for Negative where I forgot to add it. : /

Widening the return types of the Expr::*() methods isn't ideal though. I should have never made them that narrow.

Let me think about how to proceed.