Avoid desugaring non-recursive lets into recursive lets

This prepares for having linear let expressions in the frontend.

When desugaring lets, SPECIALISE statements create more copies of a
let binding. Because of the rewrite rules attached to the bindings,
there are dependencies between the generated binds.

Before this commit, we simply wrapped all these in a mutually
recursive let block, and left it to the simplified to sort it out.

With this commit: we are careful to generate the bindings in
dependency order, so that we can wrap them in consecutive lets (if the
source is non-recursive).