Chain API subsystem (#1498)

* chain-api subsystem skeleton

* chain-api subsystem: draft impl

* chain-api subsystem: mock testclient

* chain-api subsystem: impl HeaderBacked for TestClient

* chain-api subsystem: impl basic tests

* chain-api subsystem: tiny guide

* chain-api subsystem: rename ChainApiRequestMessage to ChainApiMessage

* chain-api subsystem: add the page to the ToC

* chain-api subsystem: proper error type

* chain-api subsystem: impl ancestors request

* chain-api subsystem: tests for ancestors request

* guide: fix ancestor return type

* runtime-api subsystem: remove unused dep

* fix fmt

* fix outdated comment

* chain-api subsystem: s/format/to_string

* lower-case subsystem names

* chain-api subsystem: resolve Finalized todo

* chain-api subsystem: remove TODO

* extract request errors into a module

* remove caching TODO

* fix imports

polkadot/Cargo.lock

@@ -4653,6 +4653,18 @@ dependencies = [
  "sp-keyring",
 ]
 
+[[package]]
+name = "polkadot-node-core-chain-api"
+version = "0.1.0"
+dependencies = [
+ "futures 0.3.5",
+ "maplit",
+ "polkadot-node-subsystem",
+ "polkadot-primitives",
+ "sp-blockchain",
+ "sp-core",
+]
+
 [[package]]
 name = "polkadot-node-core-proposer"
 version = "0.1.0"
@@ -4683,7 +4695,6 @@ dependencies = [
 name = "polkadot-node-core-runtime-api"
 version = "0.1.0"
 dependencies = [
- "assert_matches",
  "futures 0.3.5",
  "polkadot-node-primitives",
  "polkadot-node-subsystem",