`NoPrematureLetComputation` doesn't move multiple destructured elements

[jfmengels]

Currently the rule only reports destructured let elements when there is only a single variable being created

let
  -- is moved correctly
  (A a) = ...
  -- are not being moved
  ( b, c ) = ...
  { d } = ...
in
if doNothing then
  ()
else
  a + b + c + d

Example of things the rule would not report:

These declarations should only be moved if ALL of their variables can be moved to the same location where they can be closer to where they're being used.

let
  -- should not be moved because `b` shouldn't be moved
  ( b, c ) = ...
in
if doNothing then
  b
else
  b + c