* Enable usage of gsl::narrow with exceptions disabled

This solution uses the approach of boost::asio to enabling usage of the
library in environments where exception usage is either prohibited
or not feasible (due to code size constraints). A function template
gsl::throw_exception has been added, which in a normal environment just
throws the exception. However, when GSL_TERMINATE_ON_CONTRACT_VIOLATION
is defined the function is only declared by gsl and the definition of
this function template must be supplied by the library's user.

Closes: #468



Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

Addition:
- understand STL no exception macro
- use function static variable to set termination handler in kernel mode
- add compile-only tests for no-exception mode

* added termination tests and fixed bugs

* disabled warning C4577 for msvc 2015

- simplified storage constructors to reduce branching
- eliminated null check for constructor from array
- removed GSL_NOEXCEPT in span

* Added support for returning fixed-spize spans from subspan().

* Addressed issues from code review.

* Took simpler approach to static data member.

* Subtle fix to support MSVC 15.

* Helps to not introduce extraneous >

* fixed noexept warnings

- Removed conditional compilation for throwing version of GSL
  vs fail_fast because we don't want users of the code to see
  differences in the span interface dependent on error mechanism
  chosen
- Removed noexcept from functions that may fail at runtime
- Fixed CppCoreCheck warnings related to missing and incorrect
  noexcept
- do not warn on unnown attributes for GCC and Clang

* remove suppress that does not compiler for clang and gcc

* make explicit not_null move constructor so it is noexcept

* added test for not_null being noexcet move constructible

* rename index in multi_span to span_index

gsl::index is name reserved for different type

* add gsl::index typedef

cppcoreguidelines referece: ES.107: Don't use unsigned for subscripts, prefer gsl::index

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Res-subscripts

* rename span_index to multi_span_index

Fixing a bug in the != operator in the gsl::index class

Prevents MSVC code analysis warning
...\gsl\include\gsl\multi_span(1055): warning C26440: Function 'gsl::dim_t<-1>::dim_t<-1>' can be declared 'noexcept' (f.6: http://go.microsoft.com/fwlink/?linkid=853927).

* Use in-class init for final_action members (C.48)

Initialize `invoke_` in-class, according to http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c48-prefer-in-class-initializers-to-member-initializers-in-constructors-for-constant-initializers

* Empty commit to trigger build.

`CMAKE_VERSION_MAJOR` is just the first component of the CMake version (e.g. 3 for 3.10.1) which compares less than `3.7.9` if it is less than 3. The proper variable to compare is `CMAKE_VERSION` which contains the full version number.

Tidy up span comparison ops

Remove move ops from span

Remove from-nullptr_t constructor from span

Resolves compiler code analysis warning ...\gsl\include\gsl\string_span(101): warning C26496: The variable 'str_span' is assigned only once, mark it as const (con.4: https://go.microsoft.com/fwlink/p/?LinkID=784969).

Commit 6e2e207c renamed final_act to final_action. Do this also in
GSL.natvis.

Fixes #607.

Drive-by: use '?:' to contextually convert to bool in the fallback definition of `GSL_ASSUME`

As per feedback on span received during the C++ standardization discussions.

As per feedback during standardization discussions.

changes:
    - minimal cmake version updated to 3.7(first version with .natvis file handling)
    - updated cmake version in readme file
    - GSL.natvis file is added only to VS projects