Problems with underspecialized transform_attribute trait

[Kojoley]

Recently a lot of problems with make_attribute and transform_attribute traits were uncovered at fixing #444. I think the problem is that the trait does not have constrains, example:

spirit/include/boost/spirit/home/x3/nonterminal/detail/transform_attribute.hpp

Lines 102 to 107 in 4b2b443

	template <typename Exposed, typename Transformed>
	void post_transform(Exposed& dest, Transformed&& attr)
	{
	return transform_attribute<Exposed, Transformed, x3::parser_id>
	::post(dest, std::forward<Transformed>(attr));
	}

Because template parameter behind attr is a forwarding reference Transformed will be reference, but transform_attribute does not expect Transformed to be a reference and even const qualifier.

• Add static assertions to catch passing references and const types to Transformed parameter.
• Remove traits::pre_transform and traits::post_transform traits to be sure both pre and post uses the same transformation trait.
• Possibly forbid references in Exposed parameter.

Update: It is not clear to me if const can never appear in Transformed, so will do not touch as for now.