Add parameter category qualifier "forward" (current qualifiers are "in", "out", and "in_out") based on http://www.modernescpp.com/index.php/c-core-guidelines-how-to-pass-function-parameters. Add new usage syntax BOOST_PARAMETER_NAME((object-name), namespace-name) qualifier(tag-name)) and BOOST_PARAMETER_NAME(qualifier(name)). (Existing code that uses qualifiers directly and correctly with BOOST_PARAMETER_FUNCTION and other code generation macros should remain unaffected for now, so no breaking changes.) The reason for the change in usage is to enable applying of parameter category constraints to Boost.Parameter-enabled functions and constructors invoked through argument composition. (Otherwise, it is currently possible to use argument composition to bypass parameter category constraints applied in BOOST_PARAMETER_FUNCTION et. al.) See "test/compose.cpp" for example usage.
* Add code generation macros BOOST_PARAMETER_FUNCTION_CALL_OPERATOR and BOOST_PARAMETER_CONST_FUNCTION_CALL_OPERATOR to <boost/parameter/preprocessor.hpp>.
* Also, add documentation for BOOST_PARAMETER_CONST_MEMBER_FUNCTION, BOOST_PARAMETER_BASIC_FUNCTION, BOOST_PARAMETER_BASIC_MEMBER_FUNCTION, and BOOST_PARAMETER_BASIC_CONST_MEMBER_FUNCTION.
<boost/parameter/name.hpp>:
* Move support metafunction definitions and BOOST_PARAMETER_TAG_PLACEHOLDER_TYPE macro definition to <boost/parameter/aux_/name.hpp>.
* Move BOOST_PARAMETER_IS_BINARY macro definition to <boost/parameter/aux_/preprocessor/is_binary.hpp>.
* Use BOOST_PARAMETER_TAG_PLACEHOLDER_TYPE macro.
<boost/parameter/aux_/arg_list.hpp>:
Move forward declarations to their own header files.
<boost/parameter/aux_/tagged_argument.hpp>:
* Move boost::parameter::aux::tagged_argument_base, boost::parameter::aux::is_tagged_argument_aux, and boost::parameter::aux::is_tagged_argument to <boost/parameter/aux_/is_tagged_argument.hpp>
* Remove unnecessary forward declarations.
<boost/parameter/parameters.hpp>:
* Move support metafunction definitions to their own header files in directory boost/parameter/aux_/pack.
* Move support macro definitions to <boost/parameter/aux_/preprocessor/no_perfect_forwarding_begin.hpp> and their corresponding #undef statements to <boost/parameter/aux_/preprocessor/no_perfect_forwarding_end.hpp>.
<boost/parameter/aux_/overloads.hpp> works at the preprocessor level, so move to <boost/parameter/aux_/preprocessor/overloads.hpp>.
<boost/parameter/preprocessor.hpp>:
* Move voidstar type definition to <boost/parameter/aux_/void.hpp>.
* Move support metafunctions to their own header files in directory boost/parameter/aux_/pp_impl.
* Move support macros to header files in directory boost/parameter/aux_/preprocessor/impl.
