Cleanup.
Assume OCRA suites are strings; drop length parameters.
Begin to simplify OCRA Suite parser.
Export OCRA Suite parser, it is needed by callers to know what parameters to use.
Revert "drop parse tests for now, oath_ocra_parse_suite will not be exported anymore"

This reverts commit ebea9b13a09e17d5c44b97c02f045c682e5df5e0.
Fix OCRA Suite parser self-test.
Change challenge functions, need one to generate arbitrary challenges.
Improve challenge generation.
Simplify OCRA Suite string parser further.