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.

runtime/Cargo.toml

@@ -4,4 +4,9 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
-bytes = "1.7"
+bytes = { version = "1.7", default-features = false }
+
+[features]
+default = ["std", "alloc"]
+std = []
+alloc = []