Implement warning for shadowed imported names (-Wimport-name-shadowing)

Fixes #133 (closed)

Consider the module:

import Data.List (last)

f :: a -> a
f failed = failed

g :: a -> a
g f = f

h :: a -> a
h last = last

local :: a -> a
local x = let y = x in let x = y in x

Currently we get:

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

src/Shadow.curry:15:28 Warning:
    Shadowing symbol `x', bound at: src/Shadow.curry, line 15.7
    | 
 15 | local x = let y = x in let x = y in x
    |

With -Wimport-name-shadowing as implemented in this branch (or -Wall) we now additionally get:

src/Shadow.curry:6:3-6:8 Warning:
    Shadowing symbol `failed', bound at: /Users/fredrik/git/kics2/lib/.curry/Prelude.icurry, line 227.1
   | 
 6 | f failed = failed
   |   ^^^^^^

src/Shadow.curry:12:3-12:6 Warning:
    Shadowing symbol `last', bound at: /Users/fredrik/git/kics2/lib/.curry/Data/List.icurry, line 32.1
    | 
 12 | h last = last
    |   ^^^^

To do:

• Add flag
• Implement warning in WarnCheck
• Add test case

This warning is currently disabled by default, is this okay? (I'd argue that it's useful enough to be enabled in general, but would be fine either way)