Remove absl::result_of_t in C++20#705

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Commits on Jun 6, 2020

Used __cpp_lib_is_invocable to ifdef off all places where absl::result_of_t was used in the actual library.
obsgolem
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Fixed a couple of result_ofs I had missed in my first pass. Fixed the tests as well.
obsgolem
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Removed commented out forward-port of result_of_t.
obsgolem
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