Use namespaces

[wjwwood]

Connects to ros2/rcl#137

Examples of it in use:

% talker -t /foo/3bar
libc++abi.dylib: terminating with uncaught exception of type rclcpp::exceptions::InvalidTopicNameError: Invalid topic name: topic name token must not start with a number:
  '/foo/3bar'
        ^

[1]    82558 abort      talker -t /foo/3bar

"full" topic name check (post expansion check):

% talker -t /foo/{ns}/bar
libc++abi.dylib: terminating with uncaught exception of type rclcpp::exceptions::InvalidTopicNameError: Invalid topic name: topic name must not contain repeated '/':
  '/foo///bar'
        ^

[1]    82579 abort      talker -t /foo/{ns}/bar

Services:

% add_two_ints_client -s foo/3bar
libc++abi.dylib: terminating with uncaught exception of type rclcpp::exceptions::InvalidServiceNameError: Invalid service name: topic name token must not start with a number:
  'foo/3bar'
       ^

[1]    82599 abort      add_two_ints_client -s foo/3bar

% add_two_ints_client -s foo/3bar
libc++abi.dylib: terminating with uncaught exception of type rclcpp::exceptions::InvalidServiceNameError: Invalid service name: topic name token must not start with a number:
  'foo/3bar'
       ^

[1]    82599 abort      add_two_ints_client -s foo/3bar

Node name (locally modified):

% talker
libc++abi.dylib: terminating with uncaught exception of type rclcpp::exceptions::InvalidNodeNameError: Invalid node name: node name must not contain characters other than alphanumerics or '_':
  'invalid_talker?'
                 ^

[1]    83140 abort      talker

Namespace (locally modified):

% talker
libc++abi.dylib: terminating with uncaught exception of type rclcpp::exceptions::InvalidNamespaceError: Invalid namespace: namespace must not contain characters other than alphanumerics, '_', or '/':
  '/invalid_ns?'
              ^

[1]    83509 abort      talker

[Karsten1987]

why does this not get a prefix/suffix?

[wjwwood]

See: 2ebf066

[Karsten1987]

what is the diff between the two allocators?

[wjwwood]

Nothing right now, but they are technically different. If we were to change the default allocator for rcl versus rcutils, then this difference would be good to have. If it were someone else's code I would not require them to do future proof their code in this way.

Since it already exists, I'll leave it as-is.