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-core/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name    = "synth-core"
+version = "0.1.0"
+edition = "2021"
+description = "no_std DSP/audio components for an analogue-modelling synthesiser"
+publish = false
+
+[features]
+default = []
+std     = []
+alloc   = []
+
+[dependencies]
+libm       = { workspace = true }
+micromath  = { workspace = true }
+num-traits = { workspace = true }
+heapless   = { workspace = true }
+midi-types = { workspace = true }
+serde      = { workspace = true }
+
+[dev-dependencies]
+# approx = "0.5"