First bits of ByteString/Text parsers unification

Just enough to be able to define a single `CharParsing` instance (see ekmett/parsers#21). Here are [three](http://mvv.github.io/attoparsec/parsers-compat/bench1.html) [consecutive](http://mvv.github.io/attoparsec/parsers-compat/bench2.html) [benchmarks](http://mvv.github.io/attoparsec/parsers-compat/bench3.html) against `master` (using `PackageImports`). I didn't spot any regressions.