contracts: Allow runtime authors to define a chain extension (#7548)

* Make host functions return TrapReason

This avoids the need to manually store any trap reasons
to the `Runtime` from the host function. This adds the following
benefits:

* It properly composes with the upcoming chain extensions
* Missing to set a trap value is now a compile error

* Add chain extension

The chain extension is a way for the contract author to add new
host functions for contracts to call.

* Add tests for chain extensions

* Fix regression in set_rent.wat fixture

Not all offsets where properly updated when changing the fixtures
for the new salt on instantiate.

* Pre-charge a weight amount based off the specified length

* Improve fn write docs

* Renamed state to phantom

* Fix typo

substrate/frame/contracts/fixtures/chain_extension.wat

@@ -0,0 +1,46 @@
+;; Call chain extension by passing through input and output of this contract
+(module
+	(import "seal0" "seal_call_chain_extension"
+		(func $seal_call_chain_extension (param i32 i32 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 16 16))
+
+	(func $assert (param i32)
+		(block $ok
+			(br_if $ok (get_local 0))
+			(unreachable)
+		)
+	)
+
+	;; [0, 4) len of input output
+	(data (i32.const 0) "\02")
+
+	;; [4, 12) buffer for input
+
+	;; [12, 16) len of output buffer
+	(data (i32.const 12) "\02")
+
+	;; [16, inf) buffer for output
+
+	(func (export "deploy"))
+
+	(func (export "call")
+		(call $seal_input (i32.const 4) (i32.const 0))
+
+		;; the chain extension passes through the input and returns it as output
+		(call $seal_call_chain_extension
+			(i32.load8_u (i32.const 4))	;; func_id
+			(i32.const 4)				;; input_ptr
+			(i32.load (i32.const 0))	;; input_len
+			(i32.const 16)				;; output_ptr
+			(i32.const 12)				;; output_len_ptr
+		)
+
+		;; the chain extension passes through the func_id
+		(call $assert (i32.eq (i32.load8_u (i32.const 4))))
+
+		(call $seal_return (i32.const 0) (i32.const 16) (i32.load (i32.const 12)))
+	)
+)