Move create_inherents into the block-builder (#6553)

* Move `create_inherents` into the block-builder

This moves the `create_inherents` call into the block-builder. This has
the advantage that `create_inherents` will be able to reuse the same
context that will be used when applying the extrinsics and we also save
one call to `on_initialize`. To make sure that `create_inherents` does
not modify any state, we execute it in a transaction that is
rolled-back after doing the runtime call.

* Feedback and build fix

* Update primitives/runtime/src/lib.rs

Co-authored-by: Sergei Shulepov <sergei@parity.io>

* Update client/block-builder/src/lib.rs

Co-authored-by: Sergei Shulepov <sergei@parity.io>

substrate/client/basic-authorship/src/basic_authorship.rs

@@ -26,7 +26,6 @@ use codec::Decode;
 use sp_consensus::{evaluation, Proposal, RecordProof};
 use sp_inherents::InherentData;
 use log::{error, info, debug, trace, warn};
-use sp_core::ExecutionContext;
 use sp_runtime::{
 	generic::BlockId,
 	traits::{Block as BlockT, Hash as HashT, Header as HeaderT, DigestFor, BlakeTwo256},
@@ -200,15 +199,7 @@ impl<A, B, Block, C> Proposer<B, Block, C, A>
 			record_proof,
 		)?;
 
-		// We don't check the API versions any further here since the dispatch compatibility
-		// check should be enough.
-		for inherent in self.client.runtime_api()
-			.inherent_extrinsics_with_context(
-				&self.parent_id,
-				ExecutionContext::BlockConstruction,
-				inherent_data
-			)?
-		{
+		for inherent in block_builder.create_inherents(inherent_data)? {
 			match block_builder.push(inherent) {
 				Err(ApplyExtrinsicFailed(Validity(e))) if e.exhausted_resources() =>
 					warn!("⚠️  Dropping non-mandatory inherent from overweight block."),