Preserve original expression when parsing a formula in terms2.0?

[dmbates]

As discussed in https://github.com/JuliaInterop/RCall.jl/issues/306 I think it would be helpful to preserve a copy of the original expression passed to the @formula macro, either in the FormulaTerm type or perhaps in a Formula type that contains this expression and the FormulaTerm.

My reasons for suggesting this are twofold:

1. I have users who want to have main effects, two-factor interactions, three-factor interactions, etc. and a formula like

y ~ 1 + A * B * C * D * ...

gets expanded to

y ~ 1 + A + B + C + D + ... + A & B + A & C + ...

when I include it in the show method for the fitted model.

2. It is easier in the RCall conversion from a Julia formula to an R formula is you can just grab hold of the original expression rather than needing to reconstruct it from the lhs and the expanded rhs.

[nalimilan]

Indeed, it would make sense that when printing models we show the original formula that the user typed instead of the expanded one.