Merge Expr into RValue

[xldenis]

This finally removes the Expr type by combining it with RValue.

There is one remaining minor issue with the translation in program.rs: RValues translate to a single why3 Exp while borrows must be translated to several mlcfg statements. Either the type of RValue::to_why should be changed or maybe Borrow should be made a full statement instead?

[xldenis]

This also removes some duplicate spans around each assignment.