State machine call proof backend (#3945)

* drafting a proof extraction at the hashdb level (to include everything
for the 'call' proof case).

* use full proof by default (replace previous proof recorder).

* fix warnings.

* Cache value not found in proof recorder.

* Remove need or internal backend struct.

* fix type.

* doc

substrate/core/client/src/client.rs

@@ -18,7 +18,7 @@
 
 use std::{
 	marker::PhantomData, collections::{HashSet, BTreeMap, HashMap}, sync::Arc,
-	panic::UnwindSafe, result, cell::RefCell, rc::Rc,
+	panic::UnwindSafe, result, cell::RefCell,
 };
 use log::{info, trace, warn};
 use futures03::channel::mpsc;
@@ -1494,7 +1494,7 @@ impl<B, E, Block, RA> CallRuntimeAt<Block> for Client<B, E, Block, RA> where
 		initialize_block: InitializeBlock<'a, Block>,
 		native_call: Option<NC>,
 		context: ExecutionContext,
-		recorder: &Option<Rc<RefCell<ProofRecorder<Block>>>>,
+		recorder: &Option<ProofRecorder<Block>>,
 	) -> error::Result<NativeOrEncoded<R>> {
 		let manager = match context {
 			ExecutionContext::BlockConstruction =>