eof: Return constant hash of EXTCODEHASH of EOF

Replace runtime evaluated `keccak256("EF00")` with a constant returned
for an account with EOF code.

test/state/host.cpp

@@ -113,9 +113,18 @@ size_t Host::get_code_size(const address& addr) const noexcept
 
 bytes32 Host::get_code_hash(const address& addr) const noexcept
 {
-    // TODO: Cache code hash. It will be needed also to compute the MPT hash.
+    /// The value returned by EXTCODEHASH of an address with EOF code.
+    /// See EIP-3540: https://eips.ethereum.org/EIPS/eip-3540#changes-to-execution-semantics.
+    static constexpr auto EOF_CODE_HASH_SENTINEL =
+        0x9dbf3648db8210552e9c4f75c6a1c3057c0ca432043bd648be15fe7be05646f5_bytes32;
+
     const auto* const acc = m_state.find(addr);
-    return (acc != nullptr && !acc->is_empty()) ? keccak256(extcode(acc->code)) : bytes32{};
+    if (acc == nullptr || acc->is_empty())
+        return bytes32{};
+    if (is_eof_container(acc->code))
+        return EOF_CODE_HASH_SENTINEL;
+    // TODO: Cache code hash. It will be needed also to compute the MPT hash.
+    return keccak256(acc->code);
 }
 
 size_t Host::copy_code(const address& addr, size_t code_offset, uint8_t* buffer_data,