Tweak new list constructors.

- Remove List.new(_). I was convinced by the issue discussion that
  using it is probably a bad idea. We don't want to encourage more nulls
  in the world than there are already are. So let's see if we can live
  without it and just have List.filled(). That way users think about
  what they're creating a list *of*.
- Added some more tests.
- Correctly handle being given a negative size.

test/core/list/filled.wren

@@ -0,0 +1,8 @@
+var list = List.filled(3, "value")
+System.print(list.count) // expect: 3
+System.print(list) // expect: [value, value, value]
+
+// Can create an empty list.
+list = List.filled(0, "value")
+System.print(list.count) // expect: 0
+System.print(list) // expect: []

test/core/list/filled_size_negative.wren

@@ -0,0 +1 @@
+List.filled(-1, null) // expect runtime error: Size cannot be negative.

test/core/list/filled_size_not_int.wren

@@ -0,0 +1 @@
+List.filled(1.2, null) // expect runtime error: Size must be an integer.

test/core/list/filled_size_not_num.wren

@@ -0,0 +1 @@
+List.filled("not num", null) // expect runtime error: Size must be a number.

test/core/list/new_extra_args.wren

@@ -1,8 +0,0 @@
-var list = List.new(5)
-
-System.print(list.count) // expect: 5
-System.print(list) // expect: [null, null, null, null, null]
-
-var list2 = List.filled(5, 2)
-System.print(list2.count) // expect: 5
-System.print(list2) // expect: [2, 2, 2, 2, 2]