Fixes#510.
* Constrain the converting constructor to not participate in overload resolution when IsConst is true, so that it is never a copy constructor.
* Use Default Member Initializers for span_iterator's data members so that the default constructor can be explicitly defaulted.
* Declare all members of span_iterator GSL_NOEXCEPT: they only throw when contract violations throw.
* Don't use & in operator-> since evil types may overload it.
Casey Carter
39902b6ec0