Parachains well known keys and abridged primitives (#2194)

* Add well_known_keys

* Reorder HrmpChannel and HostConfiguration members

* abridged versions and well known keys tests

* Add some comments

* Add a note on generation of the prefixes

and other magic values

* Recommend accessing the well known values through abridged structs

polkadot/Cargo.lock

@@ -5382,6 +5382,7 @@ version = "0.8.27"
 dependencies = [
  "bitvec",
  "frame-system",
+ "hex-literal",
  "parity-scale-codec",
  "polkadot-core-primitives",
  "polkadot-parachain",
@@ -5393,6 +5394,7 @@ dependencies = [
  "sp-authority-discovery",
  "sp-core",
  "sp-inherents",
+ "sp-io",
  "sp-keystore",
  "sp-runtime",
  "sp-serializer",