[P1] reduce returns () in every arg order — appears non-functional in the VM

[ecto]

reduce is in the builtin list but does not appear to work under
loon run (0.6):

[reduce + 0 #[1 2 3]]                          ; ()
[reduce 0 + #[1 2 3]]                          ; ()
[reduce [fn [a b] [+ a b]] 0 #[1 2 3]]         ; ()

fold works correctly with [fold init fn coll] — but reduce does
not work in any arg order I tried.

Acceptance criteria

• Either fix reduce (with a clearly documented signature) or remove
  it from the builtin list and the reference. Right now the name is
  reachable but inert, which is worse than missing.