Port Queue to C++

Some knock-on changes to keep a simple design:
- Bake the data array into AVCLAN_frame_t
- Bundle the response/reaction with AVCLAN_frame_t
    - (Needed to maintain correct/simple ownership semantics with the
      cache and outgoing/incoming queues)
- Switch status frame update ticks to use a regular
  cache-originating/owned frame

src/avclan/target/avr-attiny3216/CMakeLists.txt

@@ -1,6 +1,6 @@
 # AVR / ATtiny3216 hardware target (port).
-
 include(CMakeDependentOption)
+include(FetchContent)
 
 # --- Port implementation sources -------------------------------------------
 target_sources(avclan PRIVATE
@@ -79,8 +79,21 @@ try_compile(LIBC_VERSION_TEST
     COMPILE_DEFINITIONS -mmcu=${AVR_MCU}
 )
 
+if (AVCLAN_TARGET STREQUAL "avr-attiny3216")
+FetchContent_Declare(
+    avr_libstdcpp
+    GIT_REPOSITORY https://github.com/modm-io/avr-libstdcpp.git
+    GIT_TAG 5354296040a2289c911062daa82336762231e897
+)
+FetchContent_MakeAvailable(avr_libstdcpp)
+add_library(libstdcpp INTERFACE)
+target_include_directories(libstdcpp SYSTEM
+    INTERFACE ${avr_libstdcpp_SOURCE_DIR}/include)
+target_link_libraries(avclan INTERFACE libstdcpp)
+endif()
+
+
 if(NOT LIBC_VERSION_TEST)
-    include(FetchContent)
     FetchContent_Declare(
         attiny_atpack
         URL http://packs.download.atmel.com/Atmel.ATtiny_DFP.2.0.368.atpack