bl00mbox channel callbacks

This allows users to attach callbacks to bl00mbox channels to run in the background.

Further background task functionality is planned but this one was the low hanging fruit as the intended use case is to modify the channel based on new inputs, so no lock/ownership concept is needed (compare to setting LEDs for example) and the mixer already allows bypassing them simply by muting the channel in the system mixer.

It depends on implementation details of the garbage collector since we have to play peek-a-boo with reachability in order to not leak channel memory, if a future GC returns control after marking but before sweeping this will result in a use-after-free, but we see no reason for such a change (sweeping is fast, little is gained by making it incremental) so we think it's fine.

We also took the opportunity to rewrite most of the backend, it's a bunch faster now and plugins can be garbage collected, which is technically a breaking change but it should be fine probably.

components/bl00mbox/config/bl00mbox_config.h

 #pragma once
 #define BL00MBOX_FLOW3R
 
+//#define BL00MBOX_DEBUG
+
 #ifdef BL00MBOX_FLOW3R
 //#include "flow3r_bsp.h"
 //#define BL00MBOX_MAX_BUFFER_LEN FLOW3R_BSP_AUDIO_DMA_BUFFER_SIZE
@@ -10,4 +12,18 @@
 #define BL00MBOX_LOOPS_ENABLE
 #define BL00MBOX_FREERTOS
 #define BL00MBOX_ESPIDF
+// note: this option relies on implementation details of the garbage
+// collector, specifically the fact that it is stop-the-world and
+// collects all unreachable objects it can find before it gives control
+// back to micropython. changing it from stop-the-world to anything
+// else is to our knowledge not possible without rewriting all mutators,
+// so we consider this aspect future proof. also we see no reason to
+// not collect all of them since sweeping is very fast, unless it is
+// outsourced to a separate sweeping task, in which case this option
+// may result in use-after-free, but we don't really see that happening
+// anytime soon.
+// if you want to use this option in such an environment however,
+// setting a ChannelCore's callback to None before dropping all
+// references to it solves the issue.
+#define BL00MBOX_CALLBACKS_FUNSAFE
 #endif