Better argument validation for core methods.

test/list/subscript.wren

@@ -10,16 +10,3 @@ IO.write(list[-4]) // expect: a
 IO.write(list[-3]) // expect: b
 IO.write(list[-2]) // expect: c
 IO.write(list[-1]) // expect: d
-
-// Handle out of bounds.
-// TODO: Should halt the fiber or raise an error somehow.
-IO.write(list[4])  // expect: null
-IO.write(list[-5]) // expect: null
-
-// Handle wrong argument type.
-// TODO: Should halt the fiber or raise an error somehow.
-IO.write(list[true]) // expect: null
-
-// Handle non-integer index.
-// TODO: Should halt the fiber or raise an error somehow.
-IO.write(list[1.5]) // expect: null