Lower all expressions to HIR

[fabianschuiki]

Implement lowering of all expressions defined in IEEE 1076-2008 section 9.1 to HIR. Use termification to do so, which should make handling of aggregates and other rather wild syntactic issues easier. Then go from the Term and map that to an expression. The building blocks are all there.

Make sure that hir::Expr really only accepts what the aforementioned section of the standard defines as an expression. That is, get rid of all the weird range expressions. Maybe we can drop hir::Expr and replace it directly with hir::ExprData, since the only additional information that the former carries is a span. That span can be retained via Spanned<T>.

[fabianschuiki]

This is implemented in 17b6108.