Round out the runtime

Ensures the block's size limit is respected.

Defines a policy for weights. While I'm unsure I want to commit to this
forever, I do want to acknowledge it's valid and well-defined.

Cleans up the `serai-runtime` crate a bit with further modules in the `wasm`
folder.

substrate/runtime/src/wasm/system.rs

@@ -0,0 +1,95 @@
+use super::*;
+
+/// The lookup for a SeraiAddress -> Public.
+pub struct Lookup;
+impl sp_runtime::traits::StaticLookup for Lookup {
+  type Source = SeraiAddress;
+  type Target = Public;
+  fn lookup(source: SeraiAddress) -> Result<Public, sp_runtime::traits::LookupError> {
+    Ok(source.into())
+  }
+  fn unlookup(source: Public) -> SeraiAddress {
+    source.into()
+  }
+}
+
+/// The runtime version.
+pub struct Version;
+// TODO: Are we reasonably able to prune `RuntimeVersion` from Substrate?
+impl Get<RuntimeVersion> for Version {
+  fn get() -> RuntimeVersion {
+    #[sp_version::runtime_version]
+    pub const VERSION: RuntimeVersion = RuntimeVersion {
+      spec_name: Cow::Borrowed("serai"),
+      impl_name: Cow::Borrowed("core"),
+      authoring_version: 0,
+      spec_version: 0,
+      impl_version: 0,
+      apis: RUNTIME_API_VERSIONS,
+      transaction_version: 0,
+      system_version: 0,
+    };
+    VERSION
+  }
+}
+
+impl frame_system::Config for Runtime {
+  type RuntimeOrigin = RuntimeOrigin;
+  type RuntimeCall = RuntimeCall;
+  type RuntimeEvent = RuntimeEvent;
+  type PalletInfo = PalletInfo;
+
+  type Hashing = sp_runtime::traits::BlakeTwo256;
+  type Hash = <Self::Block as sp_runtime::traits::Block>::Hash;
+
+  type Block = Block;
+  type AccountId = sp_core::sr25519::Public;
+  type Lookup = Lookup;
+  type Nonce = u32;
+
+  type PreInherents = serai_core_pallet::StartOfBlock<Runtime>;
+  type PostInherents = ();
+  type PostTransactions = serai_core_pallet::EndOfBlock<Runtime>;
+
+  /*
+    We do not globally filter the types of calls which may be performed. Instead, our ABI only
+    exposes the calls we want exposed, and each call individually errors if it's called when it
+    shouldn't be.
+  */
+  type BaseCallFilter = frame_support::traits::Everything;
+
+  /*
+    We do not have `frame_system` track historical block hashes by their block number. Instead,
+    `serai_core_pallet` populates a hash set (map of `[u8; 32] -> ()`) of all historical block's
+    hashes within itself.
+
+    The usage of `1` here is solely as `frame_system` requires it be at least `1`.
+  */
+  type BlockHashCount = ConstU64<1>;
+
+  type Version = Version;
+  type BlockLength = serai_core_pallet::Limits;
+  type BlockWeights = serai_core_pallet::Limits;
+  // We assume `serai-node` will be run using the RocksDB backend
+  type DbWeight = frame_support::weights::constants::RocksDbWeight;
+  /*
+    Serai does not expose `frame_system::Call` nor does it use transaction extensions. We
+    accordingly have no consequence to using the default weights for these accordingly.
+  */
+  type SystemWeightInfo = ();
+  type ExtensionsWeightInfo = ();
+
+  // We also don't use `frame_system`'s account system at all, leaving us to bottom these out.
+  type AccountData = ();
+  type MaxConsumers = ConstU32<{ u32::MAX }>;
+  type OnNewAccount = ();
+  type OnKilledAccount = ();
+
+  // Serai does perform any 'on-chain upgrades' to ensure upgrades are opted into by the entity
+  // running this node and accordingly consented to
+  type OnSetCode = ();
+
+  // We do not have any migrations declared
+  type SingleBlockMigrations = ();
+  type MultiBlockMigrator = ();
+}