select with optional index parameter

[KN4CK3R]

I implemented the second overload of the select operator which takes the index of the current element in the sequence.

https://msdn.microsoft.com/en-us/library/bb534869%28v=vs.110%29.aspx

Example:

std::vector<std::string> str = { "Value1", "Value2", "Value3" };
from(str)
        >> select([](auto&& s, auto&& i) { return std::make_pair(i, s); })
        >> for_each([](auto&& s) { std::cout << s.first << " = " << s.second << "\n"; });

prints

0 = Value1
1 = Value2
2 = Value3

[KN4CK3R]

added select_many too

[srvasn]

@KN4CK3R, ran into this while porting your SDK generator to ARM :)