Contracts expose pallet-xcm (#1248)

This PR introduces:
- XCM  host functions `xcm_send`, `xcm_execute`
- An Xcm trait into the config. that proxy these functions to to
`pallet_xcm`, or disable their usage by using `()`.
- A mock_network and xcm_test files to test the newly added xcm-related
functions.

---------

Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>
Co-authored-by: Sasha Gryaznov <hi@agryaznov.com>
Co-authored-by: command-bot <>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>
Co-authored-by: Alexander Theißen <alex.theissen@me.com>

substrate/bin/node/runtime/src/lib.rs

@@ -1353,6 +1353,7 @@ impl pallet_contracts::Config for Runtime {
 	type CodeHashLockupDepositPercent = CodeHashLockupDepositPercent;
 	type Debug = ();
 	type Environment = ();
+	type Xcm = ();
 }
 
 impl pallet_sudo::Config for Runtime {