Remove packrat parser

The machine parser, in pure python mode, is currently about 10% slower than the packrat parser on the JSON benchmark. In PyPy, however, the machine parser is roughly twice as fast. The Cython version is also roughly the same speed as the PyPy version. This means the packrat parser is only more performant in pure python without PyPy, so maybe it's not worth the maintenance burden of having both around.

The caveat is that the machine parser does not have memoization yet (see #15), so it will not do well on grammars where that makes a difference. I think the packrat parser can go once memoization is added to the machine parser.