sha3_256 can not be compiled

[wtd2]

In update-backend branch:

Compiler flags: -pthread -Wall -funroll-loops -Wno-ignored-attributes -Wno-unused-result -march=native -maes -mrdseed -std=c++11 -O3

Error:

emp-tool/emp-tool/circuits/bit.hpp: In constructor ‘emp::Bit_T<Wire>::Bit_T(bool, int)’:
emp-tool/emp-tool/circuits/bit.hpp:3:9: error: ‘backend’ was not declared in this scope
    3 |         backend->feed(&bit, party, &b, 1);
      |         ^~~~~~~