A size counter for Get

[copumpkin]

I've often wanted a combinator called sized :: Get a -> Get (Int, a) that will behave the same as its input, except will also tell you how many bytes it consumed.

[copumpkin]

Now that I think about it, having something similar for Put would also be nice. The use case is a fairly common one in existing binary formats: you have a discriminated/tagged union that's extensible, and implementations aren't required to understand all the tags. The typical way this is accomplished is by having a leading tag, then the size, then the constructor-specific data. I'm currently accomplishing this by pre-writing to aByteString, figuring out its size, and then writing the size and the ByteString, but that feels kind of hacky. It'd also be a natural counterpart to the sized combinator I created this ticket for. Anyone have thoughts on how to do this nicely? It also seems related to #12.

Edit: thinking about it some more, this could rekindle the distinction between Builder (which I think will eventually just be re-exported from ByteString, right?) and Put. Put would become more of a specialized State monad that knows how much has been written and can deal with alignment and write sizes cleanly. Builder would then be the major building block for that.