This fixes hard compilation error when the passed type is not an iterator
at all. As a result, path constructors from iterators are no longer
selected by the compiler in overload resolution in users' code, when
the caller is passing an initializer list with a pair of non-iterator
elements. Added a test for this fix.
Also, use integral_constant to implement boolean type traits.
Fixes https://github.com/boostorg/filesystem/issues/287.
