Embedded skipper design pattern

I like the idea of embedding a skipper rule directly in my grammar, if you
have no good reason not to.  Here's how to do it using Spirit 2.5:

1.  Define your top level grammar without a skipper:

template <typename Iterator>
struct MyGrammar : qi::grammar<Iterator, MyAttribute()>
{
    using Skipper = qi::rule<Iterator>;
    ...
}

2:  Add the skipper to the rules in your grammar that need it:

    qi::rule<IteratorTopAttribute()>  r_start;
    qi::rule<Iterator, AnotherAttribute(), Skipper>  r_definition;
    qi::rule<Iterator, ThirdAttribute()> r_something_no_skip;
    Skipper skipper;

3:  Define your rules like this:

MyGrammar<MyIterator>::MyGrammar() : MyGrammar(base_type(r_start,
"MyGrammar") {
    r_start = skip(skipper.alias()) [ * r_definition ]  >> * skipper ;
// Use this to clean up trailing whitespace
    r_start = skip(skipper.alias()) [ * r_definition ] ;    // Otherwise
use this

    skipper = qi::space ;  // or whatever you need

    r_definition = ... ;  // uses the skipper
    r_something_no_skip = ... ; // doesn't use the skipper
    ...
}

You can also use qi::lexeme or qi::no_skip where useful.

Note that the "skipper.alias()" is very necessary to avoid crashes.

Enjoy!

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