rules/optimᵒ: Simplify a rule

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
rules/optimᵒ: Make the let -> setv rule work for all cases

In case the let form had bindings that referred to previous bindings,
setting them as a tuple failed. So what we need, is to use multiple
setvs. This patch does that, and also simplifies the code of the rule a
bit.

This fixes #2.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>