Add lambda facilities.
Remove is_placeholder_expr, which is useless finally.
Add the arg<> placeholder.
Replace apply_wrap by apply.
Use lambda instead of ad-hoc solutions when possible.
Change mpl11::quote's semantics to complete mpl11::into's.
Remove detail::either, which is not useless.
Remove forward decls of operations without defaults in some concepts.
Add the mpl11::args metafunction class and placeholder.
Replace several vector_* by args<>.
Start counting at 0 in arg.