Support for phantom bindings

[lthls]

This PR restores support for generating phantom bindings.

I wrote it by enabling phantom lets, and fixing errors as I find them. Given the number of bugs I had to fix, it is likely that some remain in some less-tested corner cases.

Here is a summary of the changes:

• The Name_mode lattice is now a straight line. The typing env requires all the variables in it to have a name mode at least In_types, so I made Phantom greater than In_types instead of not comparable. It would also be possible to prevent the typing env from handling variables in Phantom mode, and that should make the environments smaller, but I think it should be a separate PR.
• The logic in Expr_builder to decide whether to keep a binding, delete it or phantomise it has been moved to Simplify_let_expr.rebuild_let. This makes decisions more consistent (now even bindings removed by mutable unboxing get phantomised if needed)
• Slot offset computations now ignore slots not in normal mode, not only in projections. Phantom closures also get Deleted code IDs to avoid keeping old code around.
• No CSE equations are created for phantom bindings

It is possible that this PR still contains changes that were made redundant by a more general fix. Do not hesitate to point these out during review.