Add zsav support

[hadley]

ZSAV was a bigger undertaking, but should be parsed seamlessly without any code changes. On the write side, set either:

readstat_writer_set_compression(writer, READSTAT_COMPRESS_BINARY);

Or

readstat_writer_set_file_format_version(writer, 3);

ZSAV support requires zlib, so some integration effort might be needed. HAVE_ZLIB needs to be defined at compile-time and the binary should be linked with -lz. No idea how that will work with Rcpp.

[lock]

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