adderall.dsl: Rename nilᵒ to emptyᵒ

In Scheme, nil is the empty list. In Hy, nil is none, so nilᵒ was a
misleading name. I will introduce a real nilᵒ once the Reasoned Schemer
tests are all done. Until then, what the Reasoned Schemer calls nilᵒ is
called emptyᵒ in Adderall.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
adderall.lvar: Minor formatting changes

Reformatted the source code to be easier to glance through it.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
adderall.internal: Use instance? instead of isinstance

This leads to more idiomatic Hy code.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
adderall.internal: Code cleanup in reify

Instead of using a let form to introduce an inner function, just use
defn. It will create a function in the local scope, which is just right
for us.

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