Support no_std in roto-runtime

Add #![no_std] to the runtime crate and introduce optional std and
alloc features. Update the code generator to be compatible and add a
no_std_test example. Remove the generator binary.

examples/no_std_test/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "no_std_test"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+roto-runtime = { path = "../../runtime", default-features = false }
+
+[profile.dev]
+panic = "abort"
+
+[profile.release]
+panic = "abort"