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.

codegen/src/bin/generator.rs

@@ -1,42 +0,0 @@
-use clap::Parser;
-use roto_codegen::generator::generate_rust_code;
-use roto_codegen::google::protobuf::descriptor::FileDescriptorSet;
-use std::fs;
-use std::path::PathBuf;
-
-#[derive(Parser)]
-#[command(
-    author,
-    version,
-    about = "Generates Rust accessor and builder code from a protobuf descriptor set"
-)]
-struct Args {
-    /// Path to the descriptor set file (.desc)
-    #[arg(short, long)]
-    input: PathBuf,
-
-    /// Path to the output directory
-    #[arg(short, long)]
-    output: PathBuf,
-
-    /// Files to generate. If omitted, all files are generated.
-    #[arg(short, long, value_delimiter = ',')]
-    files: Option<Vec<String>>,
-}
-
-fn main() -> Result<(), Box<dyn std::error::Error>> {
-    let args = Args::parse();
-    let data = fs::read(&args.input)?;
-    let set = FileDescriptorSet::new(&data).expect("Failed to parse FileDescriptorSet");
-
-    let files = generate_rust_code(&set, args.files.as_deref(), true);
-
-    for (filename, content) in files {
-        let path = args.output.join(filename);
-        if let Some(parent) = path.parent() {
-            fs::create_dir_all(parent)?;
-        }
-        fs::write(path, content)?;
-    }
-    Ok(())
-}