replace HistoricalValidationCode usages with ValidationCodeByHash (#3210)

* replace HistoricalValidationCode usages with ValidationCodeByHash

* runtime-api: tabify tests file

* update implementers guide

polkadot/roadmap/implementers-guide/src/runtime-api/historical_validation_code.md

@@ -1,7 +0,0 @@
-# Historical Validation Code
-
-Fetch the historical validation code used by a para for candidates executed in the context of a given block height in the current chain.
-
-```rust
-fn historical_validation_code(at: Block, para_id: ParaId, context_height: BlockNumber) -> Option<ValidationCode>;
-```

polkadot/roadmap/implementers-guide/src/runtime-api/validation-code.md

@@ -5,3 +5,9 @@ Fetch the validation code used by a para, making the given `OccupiedCoreAssumpti
 ```rust
 fn validation_code(at: Block, ParaId, OccupiedCoreAssumption) -> Option<ValidationCode>;
 ```
+
+Fetch the validation code (past, present or future) by its hash.
+
+```rust
+fn validation_code_by_hash(at: Block, Hash) -> Option<ValidationCode>;
+```