Catenable builder should track its length

[andrewthad]

I've tried to resist doing this because it causes more work to be done at the time that a catenable builder is constructed, but ultimately, this worth must be done as some point or another. Right now, flattening a catenable builder requires two steps:

1. Compute the total length
2. Allocate a byte array and paste the pieces into place

If every builder kept track of its own length, we could get rid of step 1. How should this change be made. Currently, Builder is defined as:

data Builder
  = Empty
  | Cons {-# UNPACK #-} !Bytes !Builder
  | Snoc !Builder {-# UNPACK #-} !Bytes
  | Append !Builder !Builder

We need something like this instead:

data Builder = Builder Int Body
data Body
  = Empty
  | Cons {-# UNPACK #-} !Bytes {-# UNPACK #-} !Builder
  | Snoc {-# UNPACK #-} !Builder {-# UNPACK #-} !Bytes
  | Append {-# UNPACK #-} !Builder {-# UNPACK #-} !Builder

That's all it should take. We unpack the length-tracking builders into the body data constructors so that we are not incurring additional indirections. In theory, we could create an unboxed newtype Builder# around a tuple of Int# and Body to guarantee that the builder type didn't cost us additional allocations. But I think GHC can probably do fine without the extra help.