Update to C++17

Highlights from #52:

Adopting C++17

• Move to PEGTL master/3.0.0 (not yet officially released)
• Use std::variant for response::Value
• Replace std::unique_ptr for nullable fields with std::optional

Schemagen improvements

• Make NYI stub generation optional
• Split the class definitions/declarations into separate files. May also help with making the NYI stubs optional (i.e. decide whether or not to compile and link them individually).
• Improve error handling/reporting.
• Add command line options for things like defining a target directory.
• Change the names of the generated Mutation accessors to something like applyFieldName. Calling everything getFieldName feels strange when performing a mutation.

Miscellaneous

• Introduce service::FieldResult which handles returning either by value of T or as a std::future<T>
• Let the caller specify the std::launch policy for the Request::resolve top-level call.
• Build a separate library target for the grammar/AST.
• Replace std::string with std::string_view where appropriate.
• Replace ast<std::string> with ast<std::vector<char>> to deal with small-string-optimization issues rather than reserving extra space when the string is too short.
• Replace the facebook::graphql namespace with just graphql for the sake of brevity.