Adding binarySize to Binary

[Musasabi]

Hello

I keep wanting something like:

binarySize :: Binary a => Proxy a -> (Int,Maybe Int)

in the Data.Binary for returning the minimum and maximum (or Nothing
for infinite) space requirements for objects of a given type.

Proxy is just defined as "data Proxy t = Proxy", but omitting it
is also possible. Would other people consider this an useful
addition?

- Einar Karttunen