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/src/main.rs

@@ -0,0 +1,16 @@
+#![no_std]
+#![no_main]
+
+use roto_runtime::ProtoAccessor;
+
+#[panic_handler]
+fn panic(_info: &core::panic::PanicInfo) -> ! {
+    loop {}
+}
+
+#[unsafe(no_mangle)]
+pub extern "C" fn _start() -> ! {
+    let _data = [0u8; 0];
+    let _ = ProtoAccessor::new(&_data);
+    loop {}
+}