Shadowing of imported symbols does not emit a warning

E.g.

f :: a -> a
f failed = failed

should warn about Prelude.failed being shadowed (similar to what GHC does when -Wall is enabled), but currently does not. Adding a function whose parameter shadows the (locally defined) function f, however, does emit a warning:

g :: a -> a
g f = f

.../src/Shadow.curry:9:3 Warning:
    Shadowing symbol `f', bound at: .../src/Shadow.curry, line 6.1
   | 
 9 | g f = f
   |   ^

Not sure if this is already covered by #62.