This is an attempt to redo the vtable work in #26 that I wound up rolling back. In the new approach, we have:

• best::dyn<Interface> is a special type for putting in a best::ptr and friends, which emulates Rust's dyn Trait.
• best::interface defines what can go in a best::dyn, which is essentially a type that provides a best::vtable plus an untyped pointer. It is expected, but not required, that this type provide helper functions that emulate calling virtual functions. The BEST_INTERFACE macro makes it "easy" to define these.
  • I tried very hard to avoid the macro, but I gave up.
• To support BEST_INTERFACE, I've added macro.h which contains all of the usual very evil macro-writing-macros.
• To support generating vtable entries, best::fnref now exposes best::fnref::fnptr, which makes it easy to convert between the member function signature (which lacks a this pointer) and the equivalent with a void* this argument.
• best::dynptr and best::dynbox are convenience aliases for working with dyns.
  • best::box now supports best::try_copy, which is needed because best::interfaces cannot guarantee copying at the moment.