Make string comparisons work with Julia v1.0

[anowacki]

Make string comparisons work on Julia v1.0. Currently we can't use === to compare due to a bug whose fix is only due to be backported in Julia v1.0.6; however there may not be any v1.0.6 at all.

• Possible solution: set a comparison via e.g.

  @static if VERSION < v1.2
      const STRING_COMPARISON_OP = :(==)
  else
      const STRING_COMPARISON_OP = :(===)
  end

  and then @eval this into the individual parse_node methods in
  src/base_types and src/derived_types.jl, and the @generated method in src/io.jl