base: Implement a Lazy<T> wrapper

base: Implement a Lazy<T> wrapper

Implement an optimized, thin wrapper that provides a convenient
mechanism for lazy initialization of the contained object on first use.
The class ensures that on-demand initialization of some expensive data
structure happens (a) in a thread-safe manner, and that (b) subsequent
checks in hot paths are cheap:

 - The class implements proper copy and move semantics inherited from
   std::optional. This is particularly desirable for our typical use
   case in deal.II where we want to initialize some payload exactly once
   (and simply copy or move it along with the container).

 - The "passive" call, i.e., when the object is already initialized is
   much cheaper than a call to std::call_once, or thread synchronization
   with a std::shared_mutex (because the initialize() or
   value_or_initialize() functions are inlined and consist of a mere
   check of a boolean followed by a conditional jump).