This code does not segfault and it compiles cleanly, and it took a while
to get here.

__::shared_pointer::make takes ownership of p the same way as shared_ptr
does: either the allocation succeeds and the returned control block owns
it, or the allocation throws and the unique_ptr frees it.

Note that this construction is safe since C++17, in which the evaluation
of constructor arguments is sequenced after a new-expression allocation.

ctl is overloaded...

- Adds is_void_v.

- Uses enable_if to disable operator* and operator[] for void types.

- Uses the new school C++-compatible atomic hack.

- There is no implicit conversion from raw pointers to unique_ptr.

Test compiles and passes now.

Just write out the constructor definitions on cppreference.com.

Make the private constructor from a raw rc into a static private method
instead of a private constructor.

Various other minor fixes.