Blocks are now just a bunch of decls followed by an atom. The type and effects
information is carried by the context -- e.g. a pi type or the other arguments
to a built-in HOF. The reason is that we're about to add decls to binders and
this way we can treat binder-decls block-decls uniformly.

Just to make the decls-in-binders update easier.

Fancy B-kinded things are a pain and they're about to get worse when we add
decls to binders. An earlier attempt at adding decls without doing this forced
me to create lots of complicated type classes to handle all the `WithExpl` and
`RolePiBinder` variants.