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.

runtime/src/lib.rs

@@ -1,5 +1,8 @@
 #![no_std]
 
+#[cfg(feature = "alloc")]
+extern crate alloc;
+
 #[cfg(feature = "std")]
 extern crate std;
 
@@ -438,6 +441,8 @@ impl<'a> Iterator for RawFieldIterator<'a> {
 #[cfg(test)]
 mod tests {
     use super::*;
+    #[cfg(feature = "alloc")]
+    use alloc::{vec, vec::{Vec}};
 
     #[test]
     fn test_varint_read_write() {