Use name AbstractFeeCurrency to avoid confusion

...with the FeeCurrency in celo/testing/FeeCurrency.sol . It also is a
good idea to clearly name abstract contracts, so that nobody tries to
use them as a non-abstract contract (e.g. tries a deployment).

packages/contracts-bedrock/src/celo/FeeCurrency.sol → packages/contracts-bedrock/src/celo/AbstractFeeCurrency.sol

@@ -3,7 +3,7 @@ pragma solidity ^0.8.15;
 
 import { ERC20 } from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
 
-abstract contract FeeCurrency is ERC20 {
+abstract contract AbstractFeeCurrency is ERC20 {
     modifier onlyVm() {
         require(msg.sender == address(0), "Only VM can call");
         _;

packages/contracts-bedrock/src/universal/OptimismMintableERC20.sol

@@ -5,15 +5,15 @@ import { ERC20 } from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
 import { IERC165 } from "@openzeppelin/contracts/utils/introspection/IERC165.sol";
 import { ILegacyMintableERC20, IOptimismMintableERC20 } from "src/universal/IOptimismMintableERC20.sol";
 import { ISemver } from "src/universal/ISemver.sol";
-import { FeeCurrency } from "src/celo/FeeCurrency.sol";
+import { AbstractFeeCurrency } from "src/celo/AbstractFeeCurrency.sol";
 
 /// @title OptimismMintableERC20
 /// @notice OptimismMintableERC20 is a standard extension of the base ERC20 token contract designed
 ///         to allow the StandardBridge contracts to mint and burn tokens. This makes it possible to
 ///         use an OptimismMintablERC20 as the L2 representation of an L1 token, or vice-versa.
 ///         Designed to be backwards compatible with the older StandardL2ERC20 token which was only
 ///         meant for use on L2.
-contract OptimismMintableERC20 is IOptimismMintableERC20, ILegacyMintableERC20, ERC20, ISemver, FeeCurrency {
+contract OptimismMintableERC20 is IOptimismMintableERC20, ILegacyMintableERC20, ERC20, ISemver, AbstractFeeCurrency {
     /// @notice Address of the corresponding version of this token on the remote chain.
     address public immutable REMOTE_TOKEN;