Issue 41 incorrect renaming

[alanz]

Closest #41

It will report a name clash for renaming x to g in each of the following

f x = let g = 'g' in x

f1 x = do
  let g = 'g'
  return x

f2 x = x
  where
    g = 'g'

f3 x = y
  where
    g = f 'g'
    y = f2 x