Add support of c++17 standard

[melg8]

When changing in CMakeLists.txt cxx standard in cmake like this:
set(CMAKE_CXX_STANDARD 17)
And compiling with:

gcc (Ubuntu 8.1.0-5ubuntu1~16.04) 8.1.0

On latest devel commit
Observe an error in choice.hpp:55:

fast_ber/ber_types/Choice.hpp:55: error: ‘VisitResult’ in namespace ‘absl::variant_internal’ does not name a template type
 absl::variant_internal::VisitResult<Visitor, Choice<Variants...>> visit(Visitor&&                  vis,
                         ^~~~~~~~~~~

I think it happens because absl fallback to std implementation of visit and variant when c++17 is enabled (look for ABSL_USES_STD_VARIANT).
So basically variant_internal::VisitResult doesn't exists in that case.

I don't know best solution, but what i can suggest:

• replace (maybe conditionally?) visit return type with decltype(auto)
• fixup places, where visit returns ostream& - because it don't like copy construction, and you need calm rvalue and lvalue references errors.

Off topic: i think it will be great to add some checks to ci - to prevent any regression in standard support.

[Samuel-Tyler]

Thank you for the report. I'll take a look at this after work.

Adding coverage for more compilers is a great idea.

[Samuel-Tyler]

I've fixed the build for 17, and added g++-7 and clang++-7 to Travis.

[melg8]

Great! Thanks for fast response.