Add no_std test example and update alloc gating

Update codegen to make `bytes` imports conditional on the `alloc`
feature and add a test example for the `thumbv7em-none-eabihf`
target.

examples/no_std_test/Cargo.toml

@@ -5,6 +5,11 @@ edition = "2024"
 
 [dependencies]
 roto-runtime = { path = "../../runtime", default-features = false }
+embedded-alloc = { version = "0.5", optional = true }
+bytes = { version = "1.7", default-features = false }
+
+[features]
+alloc = ["roto-runtime/alloc", "embedded-alloc"]
 
 [profile.dev]
 panic = "abort"