This PR has significantly reduced the number & size of string copies and subsequent heap allocations during parsing of YAML sources by the following major changes:

• use fkyaml::detail::str_view class which is newly implemented as a minimal implementation of std::string_view (since C++17) even when the library is built with C++11 or C++14 so that inspections into YAML sources are executed more effectively
• avoid copying YAML sources into a std::string object if they don't contain CR newline codes and are an array/container which has contiguous byte array to store actual characters (like char [N], std::vector or std::string)
• reduced unnecessary std::string instanciations during the parsing

As a result of the above changes, the benchmark scores of this library has increased by 0.5MiB/s (when parsing tool/benchmark/macos.yml file with the same environment described in README.md):

Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• Changes are described in the pull request or in a referenced issue.
• The test suite compiles and runs without any error.
• The code coverage on your branch is 100%.
• The documentation is updated if you added/changed a feature.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Please refrain from proposing changes that would break YAML specifications. If you propose a conformant extension of YAML to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.