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 = []

runtime/src/lib.rs

@@ -1,4 +1,9 @@
-use std::fmt;
+#![no_std]
+
+#[cfg(feature = "std")]
+extern crate std;
+
+use core::fmt;
 use bytes::BufMut;
 
 pub struct MapFieldIterator<'a> {
@@ -51,9 +56,10 @@ impl fmt::Display for RotoError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for RotoError {}
 
-pub type Result<T> = std::result::Result<T, RotoError>;
+pub type Result<T> = core::result::Result<T, RotoError>;
 
 pub trait RotoOwned {
     type Reader<'a> where Self: 'a;
@@ -686,7 +692,7 @@ mod tests {
             .iter_repeated(18)
             .map(|r| {
                 let (val, _) = r.expect("Failed to decode repeated string");
-                std::str::from_utf8(val).expect("Invalid utf8")
+                core::str::from_utf8(val).expect("Invalid utf8")
             })
             .collect();
         assert_eq!(repeated_strings, vec!["one", "two", "three"]);