Add a malloc_dyn upcall for dynamically sized allocations on the shared heap.

src/rt/rust_util.h

@@ -104,6 +104,13 @@ make_str_vec(rust_kernel* kernel, size_t nstrs, char **strs) {
     return v;
 }
 
+inline size_t get_box_size(size_t body_size, size_t body_align) {
+    size_t header_size = sizeof(rust_opaque_box);
+    // FIXME: This alignment calculation is suspicious. Is it right?
+    size_t total_size = align_to(header_size, body_align) + body_size;
+    return total_size;
+}
+
 // Initialization helpers for ISAAC RNG
 
 inline void isaac_seed(rust_kernel* kernel, uint8_t* dest)