feat: add Mempool and its API

[giladchase]

This change is 

[elintul]

Reviewed 2 of 3 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @ayeletstarkware and @giladchase)

---

crates/mempool/src/mempool.rs line 24 at r2 (raw file):

    /// Update the mempool's internal state according to the committed block's transactions.
    /// This method also resolves nonce gaps and updates account balances.

Suggestion:

/// This method also updates internal state (resolves nonce gaps, updates account balances).

[elintul]

Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @ayeletstarkware and @giladchase)

---

crates/mempool/src/mempool.rs line 19 at r2 (raw file):

    /// 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<()> {

Where, AccountState includes nonce and balance.

Suggestion:

pub fn add_tx(&mut self, tx: InternalTransaction, account_state: AccountState) -> MempoolResult<()>

---

crates/mempool/src/mempool.rs line 31 at r2 (raw file):

        block_number: u64,
        txs_in_block: &[TransactionHash],
        state_changes: StateChanges,

Suggestion:

account_states: HashMap<ContractAddress, AccountState>,

[ayeletstarkware]

Reviewed 1 of 3 files at r1, 1 of 1 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @giladchase)

[elintul]

Reviewed 1 of 1 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @giladchase)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 58.88%. Comparing base (eba70cb) to head (17d9cd1).

Files	Patch %	Lines
crates/mempool/src/mempool.rs	0.00%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
- Coverage   61.93%   58.88%   -3.06%     
==========================================
  Files          10       11       +1     
  Lines         289      304      +15     
  Branches      289      304      +15     
==========================================
  Hits          179      179              
- Misses        105      120      +15     
  Partials        5        5              

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[ayeletstarkware]

Reviewed 4 of 4 files at r4, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @giladchase)

[elintul]

Reviewed 4 of 4 files at r4, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @giladchase)