o Return its contents when necessary.  In many cases, this avoids
  construction of multiple copies of the yytext token.  Exceeds
  performance of current develop branch.

o Use std::streambuf I/O instead of std::istream; does not maintain
  (unused) istream flags.
o Further simplify get/unget handling.
o Restore original handling of NUL in input stream; ignored during
  token_string escaping.

using more CMake

Add info for the vcpkg package.