Add draft "precrime" contracts

contracts/precrime/ILayerZeroClientPrecrime.sol

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+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import { ILayerZeroClientPrecrimeInternal } from './ILayerZeroClientPrecrimeInternal.sol';
+
+interface ILayerZeroClientPrecrime is ILayerZeroClientPrecrimeInternal {
+    function precrime() external view returns (address);
+
+    function setPrecrime(address precrime) external;
+}

contracts/precrime/ILayerZeroClientPrecrimeInternal.sol

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+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+interface ILayerZeroClientPrecrimeInternal {
+    event SetPrecrime(address precrime);
+}

contracts/precrime/LayerZeroClientPrecrime.sol

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+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import { ILayerZeroClientPrecrime } from './ILayerZeroClientPrecrime.sol';
+import { LayerZeroClientPrecrimeInternal } from './LayerZeroClientPrecrimeInternal.sol';
+
+abstract contract LayerZeroClientPrecrime is
+    ILayerZeroClientPrecrime,
+    LayerZeroClientPrecrimeInternal
+{
+    function precrime() external view returns (address) {
+        return _precrime();
+    }
+
+    function setPrecrime(address precrime) external {
+        return _setPrecrime(precrime);
+    }
+}

contracts/precrime/LayerZeroClientPrecrimeInternal.sol

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+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import { ILayerZeroClientPrecrimeInternal } from './ILayerZeroClientPrecrimeInternal.sol';
+import { LayerZeroClientPrecrimeStorage } from './LayerZeroClientPrecrimeStorage.sol';
+
+abstract contract LayerZeroClientPrecrimeInternal is
+    ILayerZeroClientPrecrimeInternal
+{
+    function _precrime() internal view virtual returns (address) {
+        return LayerZeroClientPrecrimeStorage.layout().precrime;
+    }
+
+    function _setPrecrime(address precrime) internal virtual {
+        // TODO: onlyOwner
+        LayerZeroClientPrecrimeStorage.layout().precrime = precrime;
+        emit SetPrecrime(precrime);
+    }
+}

contracts/precrime/LayerZeroClientPrecrimeStorage.sol

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+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+library LayerZeroClientPrecrimeStorage {
+    struct Layout {
+        address precrime;
+    }
+
+    bytes32 internal constant STORAGE_SLOT =
+        keccak256('solidstate.contracts.storage.LayerZeroClientPrecrime');
+
+    function layout() internal pure returns (Layout storage l) {
+        bytes32 slot = STORAGE_SLOT;
+        assembly {
+            l.slot := slot
+        }
+    }
+}