PoV Reclaim (Clawback) Node Side (#1462)

This PR provides the infrastructure for the pov-reclaim mechanism
discussed in #209. The goal is to provide the current proof size to the
runtime so it can be used to reclaim storage weight.

## New Host Function
- A new host function is provided
[here](https://github.com/skunert/polkadot-sdk/blob/5b317fda3be205f4136f10d4490387ccd4f9765d/cumulus/primitives/pov-reclaim/src/lib.rs#L23).
It returns the size of the current proof size to the runtime. If
recording is not enabled, it returns 0.

## Implementation Overview
- Implement option to enable proof recording during import in the
client. This is currently enabled for `polkadot-parachain`,
`parachain-template` and the cumulus test node.
- Make the proof recorder ready for no-std. It was previously only
enabled for std environments, but we need to record the proof size in
`validate_block` too.
- Provide a recorder implementation that only the records the size of
incoming nodes and does not store the nodes itself.
- Fix benchmarks that were broken by async backing changes
- Provide new externalities extension that is registered by default if
proof recording is enabled.
- I think we should discuss the naming, pov-reclaim was more intuitive
to me, but we could also go with clawback like in the issue.

## Impact of proof recording during import
With proof recording: 6.3058 Kelem/s
Without proof recording: 6.3427 Kelem/s

The measured impact on the importing performance is quite low on my
machine using the block import benchmark. With proof recording I am
seeing a performance hit of 0.585%.

---------

Co-authored-by: command-bot <>
Co-authored-by: Davide Galassi <davxy@datawok.net>
Co-authored-by: Bastian Köcher <git@kchr.de>

substrate/primitives/api/proc-macro/src/decl_runtime_apis.rs

@@ -729,7 +729,7 @@ fn decl_runtime_apis_impl_inner(api_decls: &[ItemTrait]) -> Result<TokenStream>
 	};
 
 	let decl = expander::Expander::new("decl_runtime_apis")
-		.dry(std::env::var("SP_API_EXPAND").is_err())
+		.dry(std::env::var("EXPAND_MACROS").is_err())
 		.verbose(true)
 		.write_to_out_dir(decl)
 		.expect("Does not fail because of IO in OUT_DIR; qed");

substrate/primitives/api/proc-macro/src/impl_runtime_apis.rs

@@ -846,7 +846,7 @@ fn impl_runtime_apis_impl_inner(api_impls: &[ItemImpl]) -> Result<TokenStream> {
 	);
 
 	let impl_ = expander::Expander::new("impl_runtime_apis")
-		.dry(std::env::var("SP_API_EXPAND").is_err())
+		.dry(std::env::var("EXPAND_MACROS").is_err())
 		.verbose(true)
 		.write_to_out_dir(impl_)
 		.expect("Does not fail because of IO in OUT_DIR; qed");