Rename detail::type_list to detail::signature.
Tidy up default continuation_of implementation.
Make many executor-related functions noexcept.
Add support for nullptr_t and allocator_arg_t.

Also consolidated stdlib compatibility typedefs into header files
which are named to correspond to the equivalent stdlib header.
Add executor_work class template.
Add allocator support to executors.
Update to simplified, non-variadic executor implementation.