Typedef ptrdiff_t to ssize_t under Visual Studio. Fixes #15

[jamiebullock]

ssize_t is a POSIX extension and therefore not available under Visual Studio.

This change defines ptrdiff_t as ssize_t, which is a compatible typedef since ptrdiff_t should always be at least as big as ssize_t

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[dhess]

Thanks. I'll get to these PRs you've made in the next few days.