Change naming of rust exposed structs to ease debugging

[timsaucer]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Right now we have identical names for internal rust representations and python exposed versions. This leads to confusing debug errors like "Cannot convert Expr to Expr". It would be helpful to rename all of the internally exposed objects to something like RawExpr to avoid these confusing errors.

Describe the solution you'd like

Simple renaming - no impact otherwise.

Describe alternatives you've considered

Additional context

[Spaarsh]

The PR only changes the Expr internal struct and it is still a work in progress. I just need a review on whether this is work in the right direction or not.

[kylebarron]

fwiw I always define my classes exported to Python with a Py prefix on the rust side, and then rename the actual export from within the pyclass macro