Make a few more none_t things constexpr.

Also, line breaks for braces.
Remove use of adjacent __ in some files.

Reserved by the standard.
Default ctor for option_t<nullptr_t> should set _defined to false.
Line breaks for opening root braces.

Also two line breaks between ctors.
Fix include guards.

Briefly considered changing all project include guards to remove __s
and then decided there's no change in hell C++ will even want to use
any of the names I'm using for include guards.

So, I'm technically stomping on reserved names, but I also technically
don't really care because the chance of it coming up in this universe
is low enough.

Though I would like to add #pragma once to every header with an include
guard.