pallet-contracts: State rent fixes (#6147)

* Don't store the storage size offset in the contract itself.

* Clean the AccountDb code a bit

* Use `storage_size: 0` when creating AliveContractInfo

* Count empty storage items.

* Update frame/contracts/src/account_db.rs

Co-authored-by: Nikolay Volf <nikvolf@gmail.com>

* Use more clear wording.

Co-authored-by: Alexander Theißen <athei@users.noreply.github.com>

* Change the order of decrement and increment for storage size

Co-authored-by: Nikolay Volf <nikvolf@gmail.com>
Co-authored-by: Alexander Theißen <athei@users.noreply.github.com>

substrate/frame/contracts/fixtures/set_empty_storage.wat

@@ -0,0 +1,15 @@
+;; This module stores a KV pair into the storage
+(module
+	(import "env" "ext_set_storage" (func $ext_set_storage (param i32 i32 i32)))
+	(import "env" "memory" (memory 16 16))
+
+	(func (export "call")
+	)
+	(func (export "deploy")
+		(call $ext_set_storage
+			(i32.const 0)				;; Pointer to storage key
+			(i32.const 0)				;; Pointer to value
+			(i32.load (i32.const 0))	;; Size of value
+		)
+	)
+)