* add gsl::not_null operator<<
Hippomocks, which is a popular mocking engine for C++ uses operator<< on pointers and gets confused about gsl::not_null not having this operator.
* Update gsl
* constexpr all the things.
* remove operator=(const T&)
* it leaves *this in an invalid state if ensure_invariant fails
* implicitly converting the T to not_null and then assigning is in every way superior.
* simplify conversion from not_null<U> with constructor delegation.
* remove the converting assignment operator; again let the conversion constructor and self-assignment operator do the work.
* Cover the remaining pointer arithmetic operations as Wakely suggests in issue #447.
* Cleanup not_null conversions from null pointer constants:
* replace constructor that accepts T with constructor template that accepts U convertible to T
* remove deleted constructor that accepts int
* Attempts to initialize with nullptr, 0, 0L, 0LL, etc. all unambiguously select the deleted nullptr_t constructor.
* Nest "gsl" directory inside a new "include" directory.
* Cleanup the _MSC_VER conditionals a bit; use constexpr on VS2017+.
* Don't #define noexcept on non-Microsoft implementations.
* Workaround VS2017 bug in multi_span. (Also implement == and != for static_bounds_dynamic_range_t because I'm an EoP semantic soundness snob.)
Fixes#441.
