Previously, we would desugar for loops

for x in xs do
  # body
end

into

xs.each do |synth|
  x = synth
  # body
end

However, when the iteration variable x is used after the loop (as a captured variable inside the loop), the rewrite only becomes correct when x either has a prior assignment, or if we add a nil initialization.

This PR therefor improves the desugaring by adding a conditional initial assignment:

if not defined?(x) then
  x = nil
end

xs.each do |synth|
  x = synth
  # body
end

The rewrite also works when there are multiple iteration variables:

for x, y in xs do
  # body
end

gets desugared into

if not defined?(x) then
  x = nil
end

if not defined?(y) then
  y = nil
end

xs.each do |synth|
  x, y = synth
  # body
end