synthio: Introduce, use the AnyBiquad type

This is just `Biquad | BlockBiquad` but it's useful shorthand.

Author: jepler

shared-bindings/synthio/Note.c

@@ -42,7 +42,7 @@ static const mp_arg_t note_properties[] = {
 //|         envelope: Optional[Envelope] = None,
 //|         amplitude: BlockInput = 1.0,
 //|         bend: BlockInput = 0.0,
-//|         filter: Optional[Biquad] = None,
+//|         filter: Optional[AnyBiquad] = None,
 //|         ring_frequency: float = 0.0,
 //|         ring_bend: float = 0.0,
 //|         ring_waveform: Optional[ReadableBuffer] = None,
@@ -86,7 +86,7 @@ MP_PROPERTY_GETSET(synthio_note_frequency_obj,
     (mp_obj_t)&synthio_note_get_frequency_obj,
     (mp_obj_t)&synthio_note_set_frequency_obj);
 
-//|     filter: Optional[Biquad]
+//|     filter: Optional[AnyBiquad]
 //|     """If not None, the output of this Note is filtered according to the provided coefficients.
 //|
 //|     Construct an appropriate filter by calling a filter-making method on the

shared-bindings/synthio/__init__.c

@@ -34,6 +34,8 @@
 //|
 //|
 
+//| AnyBiquad = Biquad | BlockBiquad
+//|
 //| class EnvelopeState:
 //|     ATTACK: EnvelopeState
 //|     """The note is in its attack phase"""