XPTv8: support for informats

[evanmiller]

Two new functions:

const char *readstat_variable_get_informat(const readstat_variable_t *variable);
void readstat_variable_set_informat(readstat_variable_t *variable, const char *informat);

These only work with XPORT v8.

[evanmiller]

INFORMAT in sas7bdat is documented here BioStatMatt/sas7bdat#19

[ofajardo]

works well in pyreadstat. I will take the changes once this is merged into dev.