chore: List.filterMapM runs and returns left-to-right (#4820)

Closes #4676. Previously `List.filterMapM` was returning results
left-to-right, but evaluating right-to-left.

src/Init/Data/List/Control.lean

@@ -127,12 +127,12 @@ results `y` for which `f x` returns `some y`.
 @[inline]
 def filterMapM {m : Type u → Type v} [Monad m] {α β : Type u} (f : α → m (Option β)) (as : List α) : m (List β) :=
   let rec @[specialize] loop
-    | [],     bs => pure bs
+    | [],     bs => pure bs.reverse
     | a :: as, bs => do
       match (← f a) with
       | none   => loop as bs
       | some b => loop as (b::bs)
-  loop as.reverse []
+  loop as []
 
 /--
 Folds a monadic function over a list from left to right:

tests/lean/run/array1.lean

@@ -58,10 +58,10 @@ def tst : IO (List Nat) :=
   (if x % 2 == 0 then pure $ some (x + 10) else pure none)
 
 /--
-info: 4
-3
+info: 1
 2
-1
+3
+4
 [12, 14]
 -/
 #guard_msgs in