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

src/rt/rust_kernel.cpp

@@ -62,6 +62,11 @@ rust_kernel::malloc(size_t size, const char *tag) {
     return _region.malloc(size, tag);
 }
 
+void *
+rust_kernel::calloc(size_t size, const char *tag) {
+    return _region.calloc(size, tag);
+}
+
 void *
 rust_kernel::realloc(void *mem, size_t size) {
     return _region.realloc(mem, size);