Create R-style NA-aware parsers for reals and ints

- int32: -2147483648 (smallest integer, INT_MIN)
- real64: 0x7f f0 00 00 00 00 07 a2 (special signalling NaN)
Use NA-aware int32 and real64 parsers (fixes #18)

This allows us to detect NA values as they are being read from the
input stream.