Make Program expose the Expression

[cgettys-microsoft]

Program, at least today, appears to purely be a thin convenience wrapper around IdedExpr (Expression) - everything it can do, you can do yourself.

I have a use case that benefits from inspecting the AST (in short, memoizing that certain expressions can never evaluate certain ways - think basically partial evaluation, given these particular values, but with everything else unknown).
Now, obviously I can do without this; it just requires basically including my own version of Program in my program.
But given that everything Program exposes is already public - and therefore, anything that changes this particular field would be a semver change anyway - I don't see a great reason not to allow getting a const reference to the contained Expression. If this later changes, that's ok, it'll be no more breaking that if folks used Expression directly.