Introduce alloc feature for optional allocation

Wrap heap-allocated types and service generation in the `alloc` feature
flag to support environments without a memory allocator.

examples/no_std_test/src/main.rs

@@ -1,5 +1,5 @@
 #![no_std]
-#![no_main]
+#![cfg_attr(not(test), no_main)]
 
 mod helloworld;
 
@@ -8,10 +8,11 @@ extern crate alloc;
 
 use roto_runtime::{ProtoAccessor, RotoMessage, RotoOwned};
 
-#[cfg(feature = "alloc")]
+#[cfg(all(feature = "alloc", not(test)))]
 #[global_allocator]
 static ALLOCATOR: embedded_alloc::Heap = embedded_alloc::Heap::empty();
 
+#[cfg(not(test))]
 #[panic_handler]
 fn panic(_info: &core::panic::PanicInfo) -> ! {
     loop {}
@@ -27,6 +28,7 @@ pub extern "C" fn _critical_section_1_0_release() {}
 
 static HELLO_DATA: &[u8] = &[0x0A, 0x05, 0x57, 0x6f, 0x72, 0x6c, 0x64];
 
+#[cfg(all(not(test), not(feature = "alloc")))]
 #[unsafe(no_mangle)]
 pub extern "C" fn _start() -> ! {
     #[cfg(not(feature = "alloc"))]
@@ -58,3 +60,15 @@ pub extern "C" fn _start() -> ! {
 
     loop {}
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_helloworld_decoding() {
+        let hello = helloworld::Hello::new(HELLO_DATA).expect("failed to decode hello");
+        let name = hello.name().expect("failed to get name");
+        assert!(!name.is_empty());
+    }
+}