refactor(ctb): Make GetSigners into a library rather than contract

This required copy/pasting the SignatureDecoder code from the Safe repo.

packages/contracts-bedrock/src/Safe/GetSigners.sol

 // SPDX-License-Identifier: MIT
 pragma solidity 0.8.15;
 
-import { SignatureDecoder } from "safe-contracts/common/SignatureDecoder.sol";
+library GetSigners {
+    /// @notice Splits signature bytes into `uint8 v, bytes32 r, bytes32 s`.
+    ///         Copied directly from
+    /// https://github.com/safe-global/safe-contracts/blob/e870f514ad34cd9654c72174d6d4a839e3c6639f/contracts/common/SignatureDecoder.sol
+    /// @dev Make sure to perform a bounds check for @param pos, to avoid out of bounds access on @param signatures
+    ///      The signature format is a compact form of {bytes32 r}{bytes32 s}{uint8 v}
+    ///      Compact means uint8 is not padded to 32 bytes.
+    /// @param pos Which signature to read.
+    ///            A prior bounds check of this parameter should be performed, to avoid out of bounds access.
+    /// @param signatures Concatenated {r, s, v} signatures.
+    /// @return v Recovery ID or Safe signature type.
+    /// @return r Output value r of the signature.
+    /// @return s Output value s of the signature.
+    function signatureSplit(
+        bytes memory signatures,
+        uint256 pos
+    )
+        internal
+        pure
+        returns (uint8 v, bytes32 r, bytes32 s)
+    {
+        // solhint-disable-next-line no-inline-assembly
+        assembly {
+            let signaturePos := mul(0x41, pos)
+            r := mload(add(signatures, add(signaturePos, 0x20)))
+            s := mload(add(signatures, add(signaturePos, 0x40)))
+            /**
+             * Here we are loading the last 32 bytes, including 31 bytes
+             * of 's'. There is no 'mload8' to do this.
+             * 'byte' is not working due to the Solidity parser, so lets
+             * use the second best option, 'and'
+             */
+            v := and(mload(add(signatures, add(signaturePos, 0x41))), 0xff)
+        }
+    }
 
-abstract contract GetSigners is SignatureDecoder {
     /// @notice Extract the signers from a set of signatures.
     /// @param dataHash Hash of the data.
     /// @param signatures Signature data for identifying signers.
     /// @param requiredSignatures Amount of required valid signatures.
-    function _getNSigners(
+    function getNSigners(
         bytes32 dataHash,
         bytes memory signatures,
         uint256 requiredSignatures

packages/contracts-bedrock/src/Safe/LivenessGuard.sol

@@ -9,7 +9,7 @@ import { Enum } from "safe-contracts/common/Enum.sol";
 import { ISemver } from "src/universal/ISemver.sol";
 import { EnumerableSet } from "@openzeppelin/contracts/utils/structs/EnumerableSet.sol";
 
-contract LivenessGuard is ISemver, GetSigners, BaseGuard {
+contract LivenessGuard is ISemver, BaseGuard {
     using EnumerableSet for EnumerableSet.AddressSet;
 
     /// @notice Emitted when a new set of signers is recorded.
@@ -110,7 +110,7 @@ contract LivenessGuard is ISemver, GetSigners, BaseGuard {
 
         uint256 threshold = safe.getThreshold();
         address[] memory signers =
-            _getNSigners({ dataHash: txHash, signatures: signatures, requiredSignatures: threshold });
+            GetSigners.getNSigners({ dataHash: txHash, signatures: signatures, requiredSignatures: threshold });
 
         for (uint256 i = 0; i < signers.length; i++) {
             lastLive[signers[i]] = block.timestamp;

packages/contracts-bedrock/test/GetSigners.t.sol

@@ -8,7 +8,7 @@ import "test/safe-tools/SafeTestTools.sol";
 
 import { SignatureDecoder } from "safe-contracts/common/SignatureDecoder.sol";
 
-contract GetSigners_Test is Test, SafeTestTools, GetSigners {
+contract GetSigners_Test is Test, SafeTestTools {
     bytes4 internal constant EIP1271_MAGIC_VALUE = 0x20c13b0b;
 
     enum SigTypes {
@@ -98,7 +98,7 @@ contract GetSigners_Test is Test, SafeTestTools, GetSigners {
 
         // Recover the signatures using the _getNSigners() method.
         address[] memory gotSigners =
-            _getNSigners({ dataHash: digest, signatures: signatures, requiredSignatures: numSigs });
+            GetSigners.getNSigners({ dataHash: digest, signatures: signatures, requiredSignatures: numSigs });
 
         // Compare the list of recovered signers to the expected signers.
         assertEq(gotSigners.length, numSigs);