Changed tests and tools to use 'discard' statements instead of 'nil' for empty blocks.

tests/ambsym/mambsym1.nim

@@ -7,4 +7,4 @@ type
 proc ha() =
   var
     x: TExport # no error
-  nil
+  discard

tests/ambsym/mambsys1.nim

@@ -4,4 +4,4 @@ type
   TExport* = enum x, y, z
 
 proc foo*(x: int) =
-  nil
+  discard

tests/ambsym/mambsys2.nim

@@ -1,4 +1,4 @@
 type
   TExport* = enum x, y, z # exactly the same type!
 
-proc foo*(x: int) = nil
+proc foo*(x: int) = discard