[Certora Audit] G07. Use a mask instead of shifting left and right (safe-global#894)

This pull request includes changes to the `contracts/handler/extensible`
directory, specifically in the `MarshalLib.sol` and
`SignatureVerifierMuxer.sol` files. The changes focus on improving the
handling of data and selectors within assembly code blocks to decrease
gas usage.

Improvements to data handling and selector extraction:

*
[`contracts/handler/extensible/MarshalLib.sol`](diffhunk://#diff-7122c44132b6fc89cd7c9f3c48519c88aaf7308705a1170d307d72465eb9e1c9L41-R41):
Modified the way the `handler` is extracted from `data` by using a
bitwise AND operation to ensure proper extraction of the handler
address.
[[1]](diffhunk://#diff-7122c44132b6fc89cd7c9f3c48519c88aaf7308705a1170d307d72465eb9e1c9L41-R41)
[[2]](diffhunk://#diff-7122c44132b6fc89cd7c9f3c48519c88aaf7308705a1170d307d72465eb9e1c9L59-R59)
*
[`contracts/handler/extensible/SignatureVerifierMuxer.sol`](diffhunk://#diff-62f21ce8850527f34ef2acdacd96d4a2a1150e3e2a7e16457e82236bbd4259d2L113-R113):
Changed the extraction of `sigSelector` from `calldataload` to use a
bitwise AND operation for more accurate and secure selector extraction.

Author: remedcu

contracts/handler/extensible/MarshalLib.sol

@@ -38,7 +38,7 @@ library MarshalLib {
         assembly {
             // set isStatic to true if the left-most byte of the data is 0x00
             isStatic := iszero(shr(248, data))
-            handler := shr(96, shl(96, data))
+            handler := and(data, 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF)
         }
         /* solhint-enable no-inline-assembly */
     }
@@ -56,7 +56,7 @@ library MarshalLib {
         assembly {
             // set isStatic to true if the left-most byte of the data is 0x00
             isStatic := iszero(shr(248, data))
-            handler := shr(96, shl(96, data))
+            handler := and(data, 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF)
             selector := shl(168, shr(160, data))
         }
         /* solhint-enable no-inline-assembly */

contracts/handler/extensible/SignatureVerifierMuxer.sol

@@ -110,7 +110,7 @@ abstract contract SignatureVerifierMuxer is ExtensibleBase, ERC1271, ISignatureV
             /* solhint-disable no-inline-assembly */
             /// @solidity memory-safe-assembly
             assembly {
-                sigSelector := shl(224, shr(224, calldataload(signature.offset)))
+                sigSelector := calldataload(signature.offset)
             }
             /* solhint-enable no-inline-assembly */