Use the same types as in the real Boost.MPL.
Add *_c versions of some metafunctions that take constants.
Explicitly qualify references to mpl11::size_t to avoid any confusion with std::size_t.
Split the apply and apply_pack metafunctions.
Add support for the _all placeholder in lambdas.
Great simplification of the lambda metafunction.
Reimplement the is_placeholder_expr metafunction to be independent
of apply.
Fix the cast metafunction so it has a member value.
Remove empty line in bind.cpp.
Add the unpack_args metafunction.
Add the `protect` metafunction.
Fix lambdas in the case of nested bind expressions.
Add support for lambda expressions in the apply metafunction.
Add support for non-template "apply" member for metafunction classes.