Rename Result ok_type and err_type

[bernardnormier]

These should be success_type and failure_type, as per #648.

[InsertCreativityHere]

This was fixed in be25fbe

These names were only used internally in the compiler(s), so no where else needs fixing.
If you're adding Result to slicec-cs, make sure to pull in the latest version of slicec to get these changes.