Proposal: add `Proxy`fied versions of `bitSizeMaybe` and `finiteBitSize`

[M Farkas-Dyck]

I propose to add the following methods to `Bits` and `FiniteBits` classes:

class Bits a where
     ...

     bitSizeMaybe' :: Proxy a -> Maybe Int

class FiniteBits b where
     ...

     finiteBitSize' :: Proxy b -> Int

Rationale: working with an arbitrary `Bits a => a`, one may not have a value of type `a` as argument to `bitSizeMaybe` or `finiteBitSize`, and writing `undefined` makes me feel dirty.