Switched from unsigned long to ptrdiff_t in parser.c

Using unsigned long instead of ptrdiff_t results in non-zero
pointer deltas being stored as zero delta, giving incorrect offsets
into arrays and hence out of bounds reads.

This patch fixes the issue in all places in parser.c and adds a macro
to reduce the chances of cut-and-paste errors.

Only affects platforms where 'sizeof(long) < sizeof(size_t)' like
64-bit Windows.

See https://bugs.chromium.org/p/chromium/issues/detail?id=894933

Closes #44.