Impove Std::variant implementation

This fixes several bugs/incompatibilities and makes the code (hopefully) a little more robust and better to understand. It's the result of the discussion with @lasse.hinrichsen.

This should also fix the problem in dune-localfunctions!113 (merged)

• Remove explicit Buffer class and use aligned_storage directly
• Use a single TypeStorage implementation for all types
• Rename variant_union_ to `VariadicUnion*
• Clean up interfaces and implementation of TypeStorage and VariadicUnion
  • Only allow (default) construction of empty objects
  • Only allow move assignment. The rhs must be empty, but this can neither be checked statically nor dynamically.
  • All construction and assignment is done using special construct() and assign() methods.
  • Destruction is only done using the destruct() method.
• Impl::variant_ does no longer rely on standard construction and assignment operators (despite the two exceptions) but uses the new special methods

This fixes several bugs and incompatibilities of the old implementation:

• Non-trivial types are no longer copied binary.
• Destructors are now called properly.
• Copy/move assignment now decays to copy/move assignment of the stored value if it already has the target type.
• Unnecessary copies are avoided.