Remove use of `std::unary_function`

[h-vetinari]

This facility has been deprecated in C++11 and was removed in C++17. In particular, this means the code as-is currently doesn't compile with C++17, which can be problematic because that's often a necessary work-around for CUDA on windows, cf. e.g. https://github.com/microsoft/STL/issues/2020.

[danpovey]

Do you have time to try just removing the inheritance from std::unary_function, where it occurs, and seeing whether it affects compilation? It may not even be necessary.

mac:kaldi: git grep unary_function
misc/maintenance/cpplint.py:    ('<functional>', ('unary_function', 'binary_function',
misc/maintenance/cpplint.py:                      'pointer_to_unary_function',
src/lm/arpa-lm-compiler.cc:  struct HashType : public std::unary_function<GeneralHistKey, size_t> {
src/lm/arpa-lm-compiler.cc:  struct HashType : public std::unary_function<OptimizedHistKey, size_t> {
src/nnet3/nnet-compile-utils-test.cc:struct ComparePair : public std::unary_function<std::pair<int32, int32>, bool>
src/nnet3/nnet-compile-utils-test.cc:    public std::unary_function<std::pair<int32, int32>, bool>

[stale[bot]]

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[danpovey]

should keep this open