Implement basic elements of RMW API

[nightduck]

Implement all functions in rmw.h. This issue comprises all the simple ones, there will be others with their own issues (detailed below).

• [x] rmw_get_implementation_identifier - Returns "rmw_hazcat" string
• [x] rmw_get_serialization_format - Research this
• [x] rmw_create_node - Define node handle for this rmw (should be simple as we don't care about nodes at this level)
• [x] rmw_destroy_node - Remove handle from whatever list we stored it in above
• [x] rmw_get_serialized_message_size - measures size of serialized message
• [x] rmw_serialize - Copy from someone else, probably
• [x] rmw_deserialize - Copy from someone else
• [x] rmw_compare_gids_equal - Equal operation, One line
• [x] rmw_context_fini - Destroy context
• [x] rmw_init - Initialize middleware
• [x] rmw_shutdown - Shutdown middleware, cleanup

Unit tests in test_rmw_implementation to pass

• [x] test_init_shutdown
• [x] test_init_options
• [x] test_create_destroy_node
• [x] test_serialize_deserialize

Covered after topic ring buffer is implemented: #7

Advanced publisher/subscriber functionality: #8

Service/client/waitable/guard condition functionality: #9

Implemented when networking is done:

• [ ] rmw_publisher_assert_liveliness - Returns RMW_RET_OK until we implement networking
• [ ] rmw_publisher_wait_for_all_acked - Returns RMW_RET_OK until we implement networking

Possibly never implemented:

• [ ] rmw_service_set_on_new_request_callback - May relate to event executor
• [ ] rmw_client_set_on_new_response_callback - May relate to event executor
• [ ] rmw_event_set_callback - May relate to event executor

[nightduck]

Implemented 3 calls. Placed dummy holders for all others a67145fe7667d9e8e5f1c7f6a02d73475fdde15b

[nightduck]

2ea8feec4e1f41a5e94516b43522deea273b8cb7