Add index variable template to get index of member pointer

[uyha]

closes #60

[uyha]

@apolukhin I could not find the document on how to contribute, could you direct me on the following matters?

• How to run the tests locally? How do I add test?
• I use requires in the implementation, should I drop to SFINAE to support C++17?
• There is a case that the index cannot be found, what should I do there?

[apolukhin]

How to run the tests locally? How do I add test?

cd boost/libs/pfr/tests
../../../b2 cxxstd=17

I use requires in the implementation, should I drop to SFINAE to support C++17?

Yes, please

There is a case that the index cannot be found, what should I do there?

How about returning std::optional ? Or you could throw an exception. Or return -1. Write some samples and experiment to get the idea of which error reporting mechanic is better for that function.