Re-evaluate topic type name and type hash

[v8p1197]

Description

Fixes ros2/ros2#1682.

This PR introduces a new hash scheme based on BLAKE2.

Is this user-facing behavior change?

If users want to build messages with the new scheme, they should specify the -DROSIDL_GENERATOR_USE_RIHS02=ON flag (see below).

Additional Information

This PR can be tested in two ways:

1. Formal tests:

mkdir ~/ros2_ws/
cd ~/ros2_ws/
git clone https://github.com/v8p1197/rosidl.git src/rosidl/
colcon build
colcon test

2. Building a custom message:

mkdir ~/ros2_ws/
cd ~/ros2_ws/
git clone https://github.com/v8p1197/rosidl.git src/rosidl/
colcon build
source install/setup.bash
git clone https://github.com/v8p1197/my_test_msgs.git src/my_test_msgs/
colcon build --packages-select my_test_msgs --cmake-clean-cache --cmake-args -DROSIDL_GENERATOR_USE_RIHS02=OFF
cat build/my_test_msgs/rosidl_generator_type_description/my_test_msgs/msg/MyMessage.json | grep hash_string
# Expected output: RIHS01_aa6fa4c8d287fd38ac88d666c540b4485a93f0d6118911d513528b77f1eaf27c
colcon build --packages-select my_test_msgs --cmake-clean-cache --cmake-args -DROSIDL_GENERATOR_USE_RIHS02=ON
cat build/my_test_msgs/rosidl_generator_type_description/my_test_msgs/msg/MyMessage.json | grep hash_string
# Expected output: RIHS02_17a6e69ee67f1252c4fc5b1ab5173340

[fujitatomoya]

blake2 would be our option here, out of curiosity do we have any discussion on PMC meeting including pros/cons?

[v8p1197]

Hi @fujitatomoya, personally I don't usually take part in PMC meetings, but a discussion on this hashing scheme is reported in the referenced issue.
For sure, a pro is the shortest encoded string length.

[mjcarroll]

Neat! I think the idea was that we needed a much shorter string length so that we don't use excessive amounts of memory in resource constrained devices, as reported in the issue upstream.

Changing this hash will have some impact on previously recorded bag files, so we probably want to make sure that we get it correct and not introduce a lot of churn around that.

Rather than having discussion at the PMC meetings, I will just flag it in the next meeting so that people know to take a look at it.

[mjcarroll]

Ah, but on closer inspection, you made it opt in, so maybe that takes care of some of the immediate concern there.

[v8p1197]

Hi @mjcarroll, yes I made this flag optional but ON by default. Do you suggest changing it to OFF?

Is there anything else to do to address the referenced issue?