[Marketplace] Move `slotId()` to `Requests` library

Co-authored-by: Eric Mastro <eric.mastro@gmail.com>

contracts/Marketplace.sol

@@ -82,7 +82,7 @@ contract Marketplace is Collateral, Proofs, StateRetrieval {
     Request storage request = _request(requestId);
     require(slotIndex < request.ask.slots, "Invalid slot");
 
-    SlotId slotId = _toSlotId(requestId, slotIndex);
+    SlotId slotId = Requests.slotId(requestId, slotIndex);
     Slot storage slot = slots[slotId];
     require(slot.host == address(0), "Slot already filled");
 
@@ -399,13 +399,6 @@ contract Marketplace is Collateral, Proofs, StateRetrieval {
     return s == RequestState.New || s == RequestState.Started;
   }
 
-  function _toSlotId(
-    RequestId requestId,
-    uint256 slotIndex
-  ) internal pure returns (SlotId) {
-    return SlotId.wrap(keccak256(abi.encode(requestId, slotIndex)));
-  }
-
   function _notEqual(RequestId a, uint256 b) internal pure returns (bool) {
     return RequestId.unwrap(a) != bytes32(b);
   }

contracts/Requests.sol

@@ -42,6 +42,13 @@ library Requests {
     return RequestId.wrap(keccak256(abi.encode(request)));
   }
 
+  function slotId(
+    RequestId requestId,
+    uint256 slotIndex
+  ) internal pure returns (SlotId) {
+    return SlotId.wrap(keccak256(abi.encode(requestId, slotIndex)));
+  }
+
   function toRequestIds(
     bytes32[] memory ids
   ) internal pure returns (RequestId[] memory result) {