rules/optimᵒ: New ruleset for code optimisation

There's a single rule in the set at the moment, which when it finds that
a defn is immediately followed by a let form, will turn the let into a
setv with tuples and an array of values.

This fixes #1.

Reported-by: Paul Tagliamonte <tag@pault.ag>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>