Support writing character-compressed data in SAS files

[evanmiller]

SAS files often use RLE compression to reduce file size (this is distinct from binary compression described in #21). ReadStat can read but not write RLE-compressed files.

[evanmiller]

One potential issue is that with compressed rows, it may be impossible to predict the number of data pages required by the file, since compressed rows have variable length. This is a problem because ReadStat uses a streaming design for writing out a file, and the total page count is required at the beginning of the file.

[evanmiller]

Implemented in c6f1a0a

Enable with

readstat_writer_set_compression(writer, READSTAT_COMPRESS_ROWS);

Note that with the RLE compression algorithm, the entire file will be built in memory before being written out. This is to address the page count issue mentioned above.