chore: added benchmarks documentation

docs/BENCHMARK.md

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+# Addition of `VisitedPcs` trait
+
+Current blockifier doesn't store the complete vector of visited program counters
+for each entry point call in an invoke transaction. Instead, visited program
+counters are pushed in a `HashSet`. This is a limiting factor to perform
+profiling operations which require record of the full trace returned by
+`cairo-vm`. More flexibility is added to the blockifier with the introduction of
+trait `VisitedPcs` which allows the user to process visited program counters in
+the most suitable way for the task.
+
+## Existing code
+
+Visited program counters are kept in the `CachedState` structure as shown below:
+
+```rust
+#[derive(Debug)]
+pub struct CachedState<S: StateReader> {
+    pub state: S,
+    // Invariant: read/write access is managed by CachedState.
+    // Using interior mutability to update caches during `State`'s immutable getters.
+    pub(crate) cache: RefCell<StateCache>,
+    pub(crate) class_hash_to_class: RefCell<ContractClassMapping>,
+    /// A map from class hash to the set of PC values that were visited in the class.
+    pub visited_pcs: HashMap<ClassHash, HashSet<usize>>,
+}
+```
+
+## New code
+
+`VisitedPcs` is an additional generic parameter of `CachedState`.
+
+```rust
+#[derive(Debug)]
+pub struct CachedState<S: StateReader, V: VisitedPcs> {
+    pub state: S,
+    // Invariant: read/write access is managed by CachedState.
+    // Using interior mutability to update caches during `State`'s immutable getters.
+    pub(crate) cache: RefCell<StateCache>,
+    pub(crate) class_hash_to_class: RefCell<ContractClassMapping>,
+    /// A map from class hash to the set of PC values that were visited in the class.
+    pub visited_pcs: V,
+}
+```
+
+An implementation of the trait `VisitedPcs` is included in the blockifier with
+the name `VisitedPcsSet` and it mimics the existing `HashSet<usize>`. Also, for
+test purposes, `CachedState` is instantiated using `VisitedPcsSet`.
+
+## Performance considerations
+
+Given the importance of the blockifier in the Starknet ecosystem, we want to
+measure the performance impact of adding the trait `VisitedPcs`. The existing
+bechmark `transfers_benchmark` doesn't cover operations with `CachedState`
+therefore we need to design new ones. We have created two new benchmarks:
+
+- `cached_state_benchmark`: this benchmark tests the performance impact of
+  populating `visited_pcs` (implemented using `VisitedPcsSet`) with a realistic
+  amount of visited program counters. The size of the sets is taken from
+  transaction
+  `0x0177C9365875CAA840EA8F03F97B0E3A8EE8851A8B952BF157B5DBD4FECCB060` in the
+  mainnet. This transaction has been chosen randomly, but there is no assurance
+  that it's representative of the most common Starknet invoke transaction.
+- `execution_benchmark`: this benchmark simulates a whole invoke transaction
+  using a dummy contract.
+
+## Performance impact
+
+A script `bench.sh` has been added to benchmark the performance impact of these
+changes: it is called as
+`bash scripts/bench.sh 14e6a87722c1d0c757b1aa2756ffabe3f248fd7d e39ae0be4cec31938399199e0a1070279b4a78ed`.
+The computer running the benchmark is: Debian VM over Windows 10 with VMWare
+Workstation 17, i9-9900K, 64GB RAM, Samsung 990 Pro NVME SSD.
+
+![Benchmark results](./criterion.png)
+
+The analysis of criterion shows that there isn't statistically significant performance decrese.