Suggest optimizing `for/last` expressions

The for/last form evaluates the loop body every iteration, even though it often only needs to evaluate it on the last iteration. This is very wasteful when the loop body is computing something whose side effects don't matter. Resyntax ought to suggest something to optimize this, either by manually extracting the body out or by using a new keyword we could add to for/last that tells it to only evaluate the last loop body.

Start by tracing how resyntax recognizes for/last and how it represents optimization suggestions; the issue names no files or tests. First resolve whether the intended change is manual body extraction or a new keyword, then define completion with coverage showing the chosen transformation handles the final iteration correctly.