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/frame/contracts/fixtures/data/xcm_execute.wat

@@ -0,0 +1,52 @@
+;; This passes its input to `seal_xcm_execute` and returns the return value to its caller.
+(module
+	(import "seal0" "xcm_execute" (func $xcm_execute (param i32 i32 i32) (result i32)))
+	(import "seal0" "seal_input" (func $seal_input (param i32 i32)))
+	(import "seal0" "seal_return" (func $seal_return (param i32 i32 i32)))
+	(import "env" "memory" (memory 1 1))
+
+	;; 0x1000 = 4k in little endian
+	;; Size of input buffer
+	(data (i32.const 0) "\00\10")
+
+	(func $assert (param i32)
+		(block $ok
+			(br_if $ok
+				(get_local 0)
+			)
+			(unreachable)
+		)
+	)
+
+	(func (export "call")
+		;; Receive the encoded call
+		(call $seal_input
+			(i32.const 4)	;; Pointer to the input buffer
+			(i32.const 0)	;; Pointer to the buffer length (before call) and to the copied data length (after call)
+		)
+		;; Input data layout.
+		;; [0..4) - size of the call
+		;; [4..) - message
+
+		;; Call xcm_execute with provided input.
+		(call $assert
+			(i32.eq
+				(call $xcm_execute
+					(i32.const 4)		     ;; Pointer where the message is stored
+					(i32.load (i32.const 0)) ;; Size of the message
+					(i32.const 100)	         ;; Pointer to the where the outcome is stored
+				)
+				(i32.const 0)
+			)
+		)
+
+		(call $seal_return
+			(i32.const 0)	;; flags
+			(i32.const 100)	;; Pointer to returned value
+			(i32.const 10)	;; length of returned value
+		)
+	)
+
+	(func (export "deploy"))
+)
+