Add missing const to adjacent_filtered_range

(this is the same as #55 but in a separate branch)

I'm not sure what adjacent_filtered_excl does as it's undocumented and isn't referred to in other code. However, if it's going to persist in the code base, I think it should have this const added.

This motivating example:

#include <boost/range.hpp>
#include <boost/range/adaptor/adjacent_filtered.hpp>

#include <iostream>
#include <vector>

int main() {
    const std::vector<int> a = { 0, 1, 2, 3, 2, 1, 0 };

    a | boost::adaptors::adjacent_filtered_excl(
        [] (const int &x, const int &y) {
            return x > y;
        }
    );
}

…currently fails to compile with these GCC errors:

In file included from a.cpp:2:0:
/home/lewis/range/include/boost/range/adaptor/adjacent_filtered.hpp: In instantiation of ‘boost::range_detail::adjacent_filtered_range<BinPredicate, ForwardRng, false> boost::range_detail::operator|(const ForwardRng&, const boost::range_detail::adjacent_excl_holder<BinPredicate>&) [with ForwardRng = std::vector<int>; BinPredicate = main()::<lambda(const int&, const int&)>]’:
a.cpp:14:2:   required from here
/home/lewis/range/include/boost/range/adaptor/adjacent_filtered.hpp:190:79: error: could not convert ‘boost::range_detail::adjacent_filtered_range<main()::<lambda(const int&, const int&)>, const std::vector<int>, false>((& f)->boost::range_detail::adjacent_excl_holder<main()::<lambda(const int&, const int&)> >::<anonymous>.boost::range_detail::holder<main()::<lambda(const int&, const int&)> >::val, (* & r))’ from ‘boost::range_detail::adjacent_filtered_range<main()::<lambda(const int&, const int&)>, const std::vector<int>, false>’ to ‘boost::range_detail::adjacent_filtered_range<main()::<lambda(const int&, const int&)>, std::vector<int>, false>’
                                            const ForwardRng, false>( f.val, r );
                                                                               ^

…or these Clang errors:

In file included from a.cpp:2:
/home/lewis/range/include/boost/range/adaptor/adjacent_filtered.hpp:189:20: error: no viable conversion from returned value of type 'adjacent_filtered_range<[...], const vector<[2 * ...]>, [...]>' to function return type 'adjacent_filtered_range<[...], vector<[2 * ...]>, [...]>'
            return adjacent_filtered_range<BinPredicate,
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cpp:10:4: note: in instantiation of function template specialization 'boost::range_detail::operator|<std::__1::vector<int, std::__1::allocator<int> >, (lambda at a.cpp:11:3)>' requested here
        a | boost::adaptors::adjacent_filtered_excl(
          ^
/home/lewis/range/include/boost/range/adaptor/adjacent_filtered.hpp:110:16: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'adjacent_filtered_range<(lambda at a.cpp:11:3), const std::__1::vector<int, std::__1::allocator<int> >, false>' to 'const boost::range_detail::adjacent_filtered_range<(lambda at a.cpp:11:3), std::__1::vector<int, std::__1::allocator<int> >, false> &' for 1st argument
        struct adjacent_filtered_range
               ^
/home/lewis/range/include/boost/range/adaptor/adjacent_filtered.hpp:110:16: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'adjacent_filtered_range<(lambda at a.cpp:11:3), const std::__1::vector<int, std::__1::allocator<int> >, false>' to 'boost::range_detail::adjacent_filtered_range<(lambda at a.cpp:11:3), std::__1::vector<int, std::__1::allocator<int> >, false> &&' for 1st argument
        struct adjacent_filtered_range
               ^
/home/lewis/range/include/boost/range/iterator_range_core.hpp:236:5: note: candidate function
    operator unspecified_bool_type() const
    ^
1 error generated.

…but compiles correctly with the const added.

boostorg / range

Add missing const to adjacent_filtered_range #56