Initial commit of a toy analogue audio generator

Key components:
- no_std core so it can be used bare metal on embedded systems
- default implementation for RPi 2350, with midi input and I2s output using PIO
- Visualiser for the web to play with on a dev system

crates/synth-embedded/memory.x

@@ -0,0 +1,8 @@
+/* RP2350 (Pico 2): 2 MB XIP flash, 520 KB SRAM */
+MEMORY {
+    FLASH : ORIGIN = 0x10000000, LENGTH = 2M
+    RAM   : ORIGIN = 0x20000000, LENGTH = 520K
+}
+
+/* cortex-m-rt places the stack at the top of RAM. */
+_stack_start = ORIGIN(RAM) + LENGTH(RAM);