Proposal: Determining whether a Bits instance is FiniteBits

[Zemyla]

Every Bits instance has to know that whether it's a FiniteBits
instance as well, due to bitSizeMaybe. Therefore, it should also be
able to tell a program that it is in fact a FiniteBits instance. There
should be a function added to Data.Bits.Bits:

withFiniteBits :: Bits a => a -> (FiniteBits a => r) -> r -> r

The default should be withFiniteBits _ _ x = x, at least for the next
several versions. bitSizeMaybe can be defined as

bitSizeMaybe x = withFiniteBits x (Just (finiteBitSize x)) Nothing

once everyone is on board with properly defining the value.