use std::variant and make the dispatch functions constexpr,
avoiding runtime dispatching.

Also, deprecate the std::function<void (std::shared_ptr<MessageT>)> signature,
as it is unsafe to share one shared_ptr when multiple subscriptions take it,
because they could mutate MessageT while others are using it. So you'd have to
make a copy for each subscription, which is no different than the
std::unique_ptr<MessageT> signature or the user making their own copy in a
shared_ptr from the const MessageT & signature or the
std::shared_ptr<const MessageT> signature.

Added a std::function<void (const std::shared_ptr<const MessageT> &)> signature
to go along side the existing
std::function<void (std::shared_ptr<const MessageT>)> signature.

Removed redundant 'const' before pass-by-value signatures, e.g.
std::function<void (const shared_ptr<const MessageT>)> became
std::function<void (shared_ptr<const MessageT>)>.
This will not affect API or any users using the old style.

Signed-off-by: William Woodall <william@osrfoundation.org>

…:as_std_function<>

Signed-off-by: William Woodall <william@osrfoundation.org>

Signed-off-by: William Woodall <william@osrfoundation.org>

Signed-off-by: William Woodall <william@osrfoundation.org>

…rclcpp

Signed-off-by: William Woodall <william@osrfoundation.org>

Signed-off-by: William Woodall <william@osrfoundation.org>

Signed-off-by: William Woodall <william@osrfoundation.org>

Signed-off-by: William Woodall <william@osrfoundation.org>

…onst MessageT> & signatures.

The const std::shared_ptr<const MessageT> & signature is being kept because it
can be more flexible and efficient than std::shared_ptr<const MessageT>, but
costs realtively little to support.

The std::shared_ptr<const MessageT> signature is being kept because we want to
avoid deprecating it and causing disruption, and because it is convenient to
write, and in most cases will not materially impact the performance.

Signed-off-by: William Woodall <william@osrfoundation.org>

Signed-off-by: William Woodall <william@osrfoundation.org>

Signed-off-by: William Woodall <william@osrfoundation.org>

Signed-off-by: William Woodall <william@osrfoundation.org>

Signed-off-by: William Woodall <william@osrfoundation.org>

Signed-off-by: William Woodall <william@osrfoundation.org>

… i/o matrix

Signed-off-by: William Woodall <william@osrfoundation.org>

Signed-off-by: William Woodall <william@osrfoundation.org>

Signed-off-by: William Woodall <william@osrfoundation.org>

Signed-off-by: William Woodall <william@osrfoundation.org>