adderall.dsl: Make eitherᵍ use generators

Heavily based on and inspired by macros done by Paul Tagliamonte
<tag@pault.ag>, this turns eitherᵍ and condᵉ into lazy macros.

This allows us to drop the silly interleave implementation from
adderall.internal.

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