instead, immediately transfer ownership to MachO struct. Also, revert
back to try-ok-fail parsing approach of objects, archives, and dylibs.
It seems easier to try and fail than check if the file *is* of a
certain type given that a dylib may be a stub and parsing yaml
twice in a row seems very wasteful.

Hint for the future: if we optimise yaml/TAPI parsing, this approach
may be rethought!