allow try-runtime and TestExternalities to report PoV size (#10372)

* allow try-runtime and test-externalities to report proof size

* self review

* fix test

* Fix humanized dispaly of bytes

* Fix some test

* Fix some review grumbles

* last of the review comments

* fmt

* remove unused import

* move test

* fix import

* Update primitives/state-machine/src/testing.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* last touches

* fix

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/primitives/state-machine/src/lib.rs

@@ -188,6 +188,9 @@ mod execution {
 	/// Trie backend with in-memory storage.
 	pub type InMemoryBackend<H> = TrieBackend<MemoryDB<H>, H>;
 
+	/// Proving Trie backend with in-memory storage.
+	pub type InMemoryProvingBackend<'a, H> = ProvingBackend<'a, MemoryDB<H>, H>;
+
 	/// Strategy for executing a call into the runtime.
 	#[derive(Copy, Clone, Eq, PartialEq, Debug)]
 	pub enum ExecutionStrategy {