Added module `core.sys.darwin.mach.nlist` and `core.sys.darwin.mach.stab`

Those modules contains bindings for the 64 bits part of Mac OSX's
`<mach-o/nlish.h>` and `<mach-o/stab.h>`, respectively,
and can be used by users wishing to inspect binary data of Mach-O object files.
