feat: account_exists (#435)

Close #422

cairo/ethereum/cancun/state.cairo

@@ -425,3 +425,14 @@ func account_has_code_or_nonce{poseidon_ptr: PoseidonBuiltin*, state: State}(
     tempvar res = bool(0);
     return res;
 }
+
+func account_exists{poseidon_ptr: PoseidonBuiltin*, state: State}(address: Address) -> bool {
+    let account = get_account_optional(address);
+
+    if (cast(account.value, felt) == 0) {
+        tempvar result = bool(0);
+        return result;
+    }
+    tempvar result = bool(1);
+    return result;
+}

cairo/tests/ethereum/cancun/test_state.py

@@ -7,6 +7,7 @@
 
 from ethereum.cancun.fork_types import Address
 from ethereum.cancun.state import (
+    account_exists,
     account_has_code_or_nonce,
     get_account,
     get_account_optional,
@@ -74,6 +75,13 @@ def test_account_has_code_or_nonce(self, cairo_run, data):
         assert result_cairo == account_has_code_or_nonce(state, address)
         assert state_cairo == state
 
+    @given(data=state_and_address_and_optional_key())
+    def test_account_exists(self, cairo_run, data):
+        state, address = data
+        state_cairo, result_cairo = cairo_run("account_exists", state, address)
+        assert result_cairo == account_exists(state, address)
+        assert state_cairo == state
+
 
 class TestStateStorage:
     @given(data=state_and_address_and_optional_key(key_strategy=bytes32))