Rename ImmutableSequence to ImmutableSequenceView

Also, in doing so, drop support for unrolling iterators and making them into
indexable sequences.

See the following feedback:

  https://github.com/Textualize/textual/pull/1495#pullrequestreview-1238616797
  https://github.com/Textualize/textual/pull/1495#issuecomment-1373553580

tests/test_immutable_sequence_view.py

@@ -0,0 +1,68 @@
+import pytest
+
+from typing import Sequence
+from textual._immutable_sequence_view import ImmutableSequenceView
+
+def wrap(source: Sequence[int]) -> ImmutableSequenceView[int]:
+    """Wrap a sequence of integers inside an immutable sequence view."""
+    return ImmutableSequenceView[int](source)
+
+
+def test_empty_immutable_sequence() -> None:
+    """An empty immutable sequence should act as anticipated."""
+    assert len(wrap([])) == 0
+    assert bool(wrap([])) is False
+    assert list(wrap([])) == []
+
+
+def test_non_empty_immutable_sequence() -> None:
+    """A non-empty immutable sequence should act as anticipated."""
+    assert len(wrap([0])) == 1
+    assert bool(wrap([0])) is True
+    assert list(wrap([0])) == [0]
+
+
+def test_no_assign_to_immutable_sequence() -> None:
+    """It should not be possible to assign into an immutable sequence."""
+    tester = wrap([1,2,3,4,5])
+    with pytest.raises(TypeError):
+        tester[0] = 23
+    with pytest.raises(TypeError):
+        tester[0:3] = 23
+
+
+def test_no_del_from_iummutable_sequence() -> None:
+    """It should not be possible delete an item from an immutable sequence."""
+    tester = wrap([1,2,3,4,5])
+    with pytest.raises(TypeError):
+        del tester[0]
+
+
+def test_get_item_from_immutable_sequence() -> None:
+    """It should be possible to get an item from an immutable sequence."""
+    assert wrap(range(10))[0] == 0
+    assert wrap(range(10))[-1] == 9
+
+
+def test_get_slice_from_immutable_sequence() -> None:
+    """It should be possible to get a slice from an immutable sequence."""
+    assert list(wrap(range(10))[0:2]) == [0,1]
+    assert list(wrap(range(10))[0:-1]) == [0,1,2,3,4,5,6,7,8]
+
+
+def test_immutable_sequence_contains() -> None:
+    """It should be possible to see if an immutable sequence contains a value."""
+    tester = wrap([1,2,3,4,5])
+    assert 1 in tester
+    assert 11 not in tester
+
+
+def test_immutable_sequence_index() -> None:
+    tester = wrap([1,2,3,4,5])
+    assert tester.index(1) == 0
+    with pytest.raises(ValueError):
+        _ = tester.index(11)
+
+
+def test_reverse_immutable_sequence() -> None:
+    assert list(reversed(wrap([1,2]))) == [2,1]