Make ByteString Buffer safe against reuse
Make Text Buffer safe against reuse

--HG--
rename : Data/Attoparsec/ByteString/Fhthagn.hs => Data/Attoparsec/Internal/Fhthagn.hs
pappend for ByteString
Back out the repeated-append-to-same-Buffer changes

I'm torn over this. The use-counting changes obviously improve
safety, but they do so for a case that should never arise in practice,
and they come with a *steep* cost in performance (many benchmarks
regress to slower than 0.11).

I am inclined to either try to find a near-zero-cost alternative
that preserves safety, or to document the API as "this is a single-use
continuation, so don't call it more than once".
Fix IResult docs
IResult does not need to be parameterised on internal state