perf(generator): use memcpy paths for primitive sequence conversion (#26)

For UnboundedSequence<BasicType> fields, route both conversion
directions through single-memcpy paths instead of element-by-element
loops.

- from_rmw_message: emit msg.field.into() instead of
  msg.field.into_iter().collect(). Routes through rosidl_runtime_rs'
  Copy-bounded From<Sequence<T>> for Vec<T> -> as_slice().to_vec() ->
  memcpy. The old path paid a per-element read + zero-write through
  SequenceIterator::next().

- into_rmw_message Owned arm: emit msg.field.as_slice().into() instead
  of msg.field.into(). Routes through From<&[T]> for Sequence<T> ->
  Sequence::new + clone_from_slice -> memcpy (stdlib specializes
  clone_from_slice for T: Copy). The old path went through
  From<Vec<T>> -> from_iter -> extend, paying a per-element IndexMut
  write.

The Borrowed arm of into_rmw_message already used as_slice().into() and
is unchanged. Non-primitive element types (String, WString, nested
messages) still use .into_iter().map(...).collect() -- they need real
per-element conversion regardless.

Also restructures the from_rmw_message UnboundedSequence branch so each
element-type arm emits a single complete expression rather than sharing
a chained .into_iter()...collect() across arms.

Microbenchmarks (Sequence<u8> 65536 <-> Vec<u8>):
  receive:  23.3 us -> 513 ns (45.4x)
  publish:  28.3 us -> 726 ns (39.0x)

End-to-end (UInt8MultiArray, single-process, reliable QoS,
rmw_fastrtps_cpp, 64 KiB, median of 3 x 5s):
  rclrs (main):            30,578 msg/s
  rclrs (this branch):    135,560 msg/s  (4.4x over main, ~90% of rclcpp)
  rclcpp baseline:        151,279 msg/s
  rclrs + Worker sub:     224,576 msg/s  (~1.48x of rclcpp single-process)

Requires rosidl_runtime_rs with the new Copy-bounded
From<Sequence<T>> for Vec<T> impl (companion PR).

Refs ros2-rust/ros2_rust#628 improvement (1).

Author: azerupi

rosidl_generator_rs/resource/templates/msg_idiomatic.rs.em

@@ -139,8 +139,12 @@ impl rosidl_runtime_rs::Message for @(type_name) {
           .into_iter()
           .map(|elem| @(get_rs_type(member.type.value_type))::into_rmw_message(std::borrow::Cow::Owned(elem)).into_owned())
           .collect(),
+@# Owned BasicType sequences: borrow the slice and route through
+@# From<&[T]> for Sequence<T> so the copy compiles to a memcpy via
+@# clone_from_slice's Copy specialization, instead of From<Vec<T>>'s
+@# per-element extend path. See ros2-rust/ros2_rust#628.
 @[        else]@
-        @(get_rs_name(member.name)): msg.@(get_rs_name(member.name)).into(),
+        @(get_rs_name(member.name)): msg.@(get_rs_name(member.name)).as_slice().into(),
 @[        end if]@
 @#
 @#
@@ -243,14 +247,26 @@ impl rosidl_runtime_rs::Message for @(type_name) {
 @#
 @#    == UnboundedSequence ==
 @[    elif isinstance(member.type, UnboundedSequence)]@
+@# Primitive sequences use the From<Sequence<T>> for Vec<T> impl (memcpy via
+@# as_slice().to_vec()), avoiding O(n) per-element read/zero-write that the
+@# SequenceIterator path performs. See ros2-rust/ros2_rust#628.
+@[        if isinstance(member.type.value_type, BasicType)]@
+      @(get_rs_name(member.name)): msg.@(get_rs_name(member.name)).into(),
+@[        elif isinstance(member.type.value_type, UnboundedString) or isinstance(member.type.value_type, UnboundedWString)]@
       @(get_rs_name(member.name)): msg.@(get_rs_name(member.name))
           .into_iter()
-@[        if isinstance(member.type.value_type, UnboundedString) or isinstance(member.type.value_type, UnboundedWString)]@
           .map(|elem| elem.to_string())
+          .collect(),
 @[        elif isinstance(member.type.value_type, NamedType) or isinstance(member.type.value_type, NamespacedType)]@
+      @(get_rs_name(member.name)): msg.@(get_rs_name(member.name))
+          .into_iter()
           .map(@(get_rs_type(member.type.value_type))::from_rmw_message)
-@[        end if]@
           .collect(),
+@[        else]@
+      @(get_rs_name(member.name)): msg.@(get_rs_name(member.name))
+          .into_iter()
+          .collect(),
+@[        end if]@
 @#
 @#
 @#    == UnboundedString + UnboundedWString ==