std::result_of removed in C++20

[maxgolov]

Fix for #104 : when OpenTelemetry is compiled with latest update of Visual Studio 2019 in C++17 mode - it'd generate a warning, and in C++20 it won't compile because std::result_of has been removed in C++20. Adjust the header to stop using a compiler feature that's been removed from standard.

[reyang]

LGTM.

[g-easy]

Failing test:

[ RUN      ] RandomTest.GenerateRandomBuffer
sdk/test/common/random_test.cc:30: Failure
Value of: std::equal(std::begin(buf1), std::end(buf1), std::begin(buf2))
  Actual: true
Expected: false
[  FAILED  ] RandomTest.GenerateRandomBuffer (0 ms)

cc @pyohannes

[reyang]

@maxgolov please rebase, thanks.