Make find() constexpr in C++11

[eyalroz]

Currently, the find() method of basic_string_view is nssv_constexpr14. It seems to me it should be possible to make it constexpr since C++11 with tail-recursion iteration.

[martinmoene]

I'd like to do so, can you hint on an implementation?

[eyalroz]

@martinmoene : Is the naive implementation not ok for the constexpr case?

[eyalroz]

@martinmoene : See #49 .

[martinmoene]

@eyalroz thanks for your input! It helps to get me going :)

[eyalroz]

@martinmoene : Do you think we could possibly use an immediately-invoked lambda instead of the function call?

[martinmoene]

Not immediately ;)

Can you explain what the driving idea behind that is?

In C++14, one can create a recursive lambda, but not immediately invoke it.

[eyalroz]

Right, we wouldn't be able to recurse. I just wanted to avoid the separate function outside the class, if it's only used in one place.