rename data types and functions

[sfultong]

This issue can serve as a way to reference renamings that should be done. If you find anything that has a confusing/bad name, please put it here.

Things like IExpr, Term1, DataType, ZeroType, etc.

[hhefesto]

Term2, Term3, Term4

I would argue in favor of DataType and ZeroType (maybe also IExpr, I've gotten used to it). But not vehemently argue, just ...

They are descriptive (maybe not IExpr)

[sfultong]

Yeah, ZeroType is alright, I guess. In one branch I once renamed it to NoFunctionsType

[sfultong]

Or something like that