Implement multi-interface best::dyns

[mcy]

This change makes best::dyn a variadic template, so that you can have e.g. best::dynptr<reader, writer>, much like Rust has &dyn Read + Write. To enable this, the interface for implementing interfaces has changed.

• best::itable is the new name for best::vtable, which in turn is now an alias for extracting a BestVtable from an interface type, which is the new name for BestFuncs.
• BestImplements now needs to return a best::vtable<I> by value, and machinery inside of best::itable will automatically create a global variable for the result; best::itable::of now does more than just call BestImplements. This makes BestImplements very painless to implement by hand.
• best::itable<I, J, ...> no longer has an operator->, since there is more than one vtable; operator[] can be used to get the vtable for a particular interface.
• best::dynptr<I, J, ...> provides access to the call interfaces for I and J via operator[] too.
• best::dyn::of is now best::as_dyn (which itself was renamed to best::dyn_of). This is so that it can be called with const interface values.
• All interfaces must derive best::interface_base, which allows us to force all interfaces to provide an of() static function, instead of having ti be convention.

Also fixed best::row::as_ref() being completely broken. Oops.