* This makes it possible to use promise_base for optional
promises directly rather than wrapping it as optional<promise_base<...>>.
* Invalidate the promise_base after its first usage.
* Expose an `operator bool()` to make the validility accessible.
* Remove the no longer needed private promise_no_init_arg_t tag.
* Makes compiler output much more readable
This is configurateable through CONTINUABLE_WITH_IMMEDIATE_TYPES,
and automatically enabled for debug builds but disabled
for release builds.
* Remove the old continuable-trait.hpp header
* Make the signature of continuable not dependent anymore
on any size of the arguments which fixes the compilation
with forward declared types.
Thanks Rogiel for the correspoding bug report.
Closes#11
