Replace with std::string_view for C++17

[gabyx]

No description provided.

[gabyx]

unknown warning group '-Wself-assign-overloaded', ignored
I needed to disable this warning in the test: need to turn that on for clang greater then a specific version...
for clang 7.0 it does not work.

[coveralls]

Coverage decreased (-0.03%) to 92.254% when pulling d4f31cd on gabyx:feature/remove-string-view into b3a131c on rttrorg:master.

[gabyx]

I included a cmake options USE_LATEST_STD which make the library if MAX_CXX_VERSION is >= 17 using std::string_view which adjusts the minimum version to 17.

[mapron]

Oh, here ia the answer why RTTR uses own string_view. Any progress on this MR?