emitter: Support std::string_view

[dendy]

Accept Emitter::operator<<(std::string_view).

ABI remains C++11 compatible by exposing new method Emitter::Write(const char*, size_t).

All affected calls optimized to pass std::string values as pointer + size tuple into appropriate routines.

[jbeder]

This all looks good. Can you add a test case that emits a string_view?

[dendy]

@jbeder Could you please take a look.

1. Since Emitter's operator<< are inlined it is safe to direct std::string and const char* overloads to the new Write::(const char*, size_t). Older applications will preserve binary compatibility.

2. Fixed comparing null literals.

3. Fixed calling RexEx::Matches and added static_assert() to check for accidental passing raw C-string into the Matches.

4. Added 3 Emitter tests: for std::string, null terminated and unterminated std::string_view.

All tests are passing at my side when I configure yaml-cpp with extra option CMAKE_CXX_STANDARD=17.