Add structure-free read_multiple_bytes_packed

Motivation: A server sends data packages that consist of multiple
serialized capnproto messages of different structures. Every message is
guaranteed to have the same first field, which works as a message header
containing information about the message structure type.
The scheme comprises the `UnknownMessage` structure that allows parsing
the header only.

Solution: provide a public interface that iterates buffer with
AnyPointer readers to cast a message to `UnknownMessage` first and then
to a specific structure type.

test/test_structs_sequence.py

@@ -0,0 +1,93 @@
+import os
+
+import pytest
+
+import capnp
+
+this_dir = os.path.dirname(__file__)
+
+
+@pytest.fixture
+def message_schemas():
+    return capnp.load(os.path.join(this_dir, "test_structs_sequence.capnp"))
+
+
+@pytest.fixture
+def make_apple(message_schemas):
+    def _make_apple(color: str):
+        apple = message_schemas.Apple.new_message()
+        apple.fruitId = message_schemas.FruitId.apple
+        apple.color = color
+        return apple
+
+    return _make_apple
+
+
+@pytest.fixture
+def red_apple(make_apple):
+    return make_apple("Red")
+
+
+@pytest.fixture
+def green_apple(make_apple):
+    return make_apple("Green")
+
+
+@pytest.fixture
+def banana(message_schemas):
+    banana_ = message_schemas.Banana.new_message()
+    banana_.fruitId = message_schemas.FruitId.banana
+    banana_.length = 12.345
+    return banana_
+
+
+@pytest.fixture
+def cherry(message_schemas):
+    cherry_ = message_schemas.Cherry.new_message()
+    cherry_.fruitId = message_schemas.FruitId.cherry
+    cherry_.sweetness = 64
+    return cherry_
+
+
+@pytest.fixture
+def fruit_basket(cherry, red_apple, banana, green_apple):
+    return [cherry, red_apple, banana, green_apple]
+
+
+@pytest.fixture
+def fruit_basket_encoded(fruit_basket):
+    return b"".join(fruit.to_bytes_packed() for fruit in fruit_basket)
+
+
+@pytest.fixture
+def expected(fruit_basket):
+    return [fruit.to_dict() for fruit in fruit_basket]
+
+
+def test_parse_structs_sequence(message_schemas, fruit_basket_encoded, expected):
+    # ARRANGE
+    reader = capnp.read_multiple_bytes_packed(fruit_basket_encoded)
+
+    def _parse_fruit(any_):
+        unknown_fruit = any_.as_struct(message_schemas.UnknownFruit)
+        if unknown_fruit.fruitId == message_schemas.FruitId.apple:
+            return any_.as_struct(message_schemas.Apple)
+
+        if unknown_fruit.fruitId == message_schemas.FruitId.banana:
+            return any_.as_struct(message_schemas.Banana)
+
+        if unknown_fruit.fruitId == message_schemas.FruitId.cherry:
+            return any_.as_struct(message_schemas.Cherry)
+
+        return unknown_fruit
+
+    # ACT
+    parsed = [_parse_fruit(any_).to_dict() for any_ in reader]
+
+    # ASSERT
+    assert parsed == expected
+
+
+def test_empty_sequence():
+    reader = capnp.read_multiple_bytes_packed(b"")
+    assert len(list(reader)) == 0