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/build.rs

@@ -0,0 +1,10 @@
+use std::{env, fs, path::PathBuf};
+
+fn main() {
+    println!("cargo:rerun-if-changed=memory.x");
+
+    // Copy memory.x into OUT_DIR so the `-Tmemory.x` linker flag can find it.
+    let out = PathBuf::from(env::var("OUT_DIR").unwrap());
+    fs::copy("memory.x", out.join("memory.x")).unwrap();
+    println!("cargo:rustc-link-search={}", out.display());
+}