Contracts: Memory and Module instance teardowns (#171)

* Add teardowns.

* Rebuild binaries.

substrate/substrate/runtime-sandbox/without_std.rs

@@ -76,9 +76,12 @@ mod ffi {
 			val_ptr: *const u8,
 			val_len: usize,
 		) -> u32;
-
-	// TODO: ext_instance_teardown
-	// TODO: ext_memory_teardown
+		pub fn ext_sandbox_memory_teardown(
+			memory_idx: u32,
+		);
+		pub fn ext_sandbox_instance_teardown(
+			instance_idx: u32,
+		);
 	}
 }
 
@@ -122,6 +125,14 @@ impl Memory {
 	}
 }
 
+impl Drop for Memory {
+	fn drop(&mut self) {
+		unsafe {
+			ffi::ext_sandbox_memory_teardown(self.memory_idx);
+		}
+	}
+}
+
 pub struct EnvironmentDefinitionBuilder<T> {
 	env_def: sandbox_primitives::EnvironmentDefinition,
 	_marker: marker::PhantomData<T>,
@@ -265,3 +276,11 @@ impl<T> Instance<T> {
 		}
 	}
 }
+
+impl<T> Drop for Instance<T> {
+	fn drop(&mut self) {
+		unsafe {
+			ffi::ext_sandbox_instance_teardown(self.instance_idx);
+		}
+	}
+}