* Added support of not_null<smart_ptr> comparison
* The return type of not_null comparison operators is determined using SFINAE
#474
* tests for gsl::not_null comparison were added
* not_null comparison tests were rewritten to compare pointers to objects located in the same array
* not_null<shared_ptr> comparison was simplified
* initializer_list overload returns by value to avoid lifetime issues
* generic overload uses expression SFINAE to work with any type that has member size() and operator[], which notably includes const/non-const vector and array.
* Add test coverage for const objects, rvalue initializer_lists, and constexpr usage.
Fixes#357.
* Improve const correctness in string_span
* Improve const correctness in bounds_tests.cpp and byte_tests.cpp
* Improve const correctness in span_tests.cpp
* Improve const correctness in utils_tests.cpp
* Use gsl::owner for dynamically allocated memory in string_span_tests.cpp
* Improve const correctness in string_span_tests.cpp
* Improve const correctness for strided_span_tests.cpp
* GSL_LIKELY/_UNLIKELY always contextually convert their argument to bool.
* Add macro GSL_ASSUME(cond) to inform the optimizer that the predicate cond must hold.
* Reimplement Expects & Ensures as expressions for flexibility and correctness. (Think about "if (cond1) Expects(cond2); else goto fail;")
* Expects and Ensures GSL_ASSUME their predicate when GSL_UNENFORCED_ON_CONTRACT_VIOLATION is defined.
* 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.
