feat: add Mempool and its API

crates/mempool/src/errors.rs

@@ -0,0 +1,2 @@
+#[derive(Debug, Error)]
+pub enum MempoolError {}

crates/mempool/src/lib.rs

@@ -1,2 +1,4 @@
 // TODO: change to pub(crate) once this is used by the (not yet implemented) mempool struct.
+pub mod errors;
+pub mod mempool;
 pub mod priority_queue;

crates/mempool/src/mempool.rs

@@ -0,0 +1,37 @@
+use starknet_api::{internal_transaction::InternalTransaction, transaction::TransactionHash};
+
+use crate::errors::MempoolError;
+
+pub type MempoolResult<T> = Result<T, MempoolError>;
+
+pub struct Mempool;
+
+impl Mempool {
+    /// Retrieves up to `n_txs` transactions with the highest priority from the mempool.
+    /// Transactions are guaranteed to be unique across calls until `commit_block` is invoked.
+    // TODO: the last part about commit_block is incorrect if we delete txs in get_txs and then push back.
+    pub fn get_txs(n_txs: u8) -> MempoolResult<Vec<InternalTransaction>> {
+        todo!();
+    }
+
+    /// Adds a new transaction to the mempool.
+    /// TODO: support fee escalation and transactions with future nonces.
+    pub fn add_tx(&mut self, tx: InternalTransaction) -> MempoolResult<()> {
+        todo!();
+    }
+
+    /// Update the mempool's internal state according to the committed block's transactions.
+    /// This method also resolves nonce gaps and updates account balances.
+    // TODO: the part about resolving nonce gaps is incorrect if we delete txs in get_txs and then
+    // push back.
+    pub fn commit_block(
+        &mut self,
+        block_number: u64,
+        txs_in_block: &[TransactionHash],
+        state_changes: StateChanges,
+    ) -> MempoolResult<()> {
+        todo!()
+    }
+}
+
+pub struct StateChanges;