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

.cargo/config.toml

@@ -0,0 +1,8 @@
+[target.wasm32-unknown-unknown]
+rustflags = [
+    "-C", "target-feature=+simd128",
+]
+
+# Bare-metal Cortex-M4F (e.g. Daisy Seed / STM32H750) — uncomment when needed:
+# [target.thumbv7em-none-eabihf]
+# rustflags = ["-C", "link-arg=-Tlink.x"]