Support two-level namespace

[kastiglione]

After reviewing #31, it seems a piece of missing functionality in fishhook is support for mach-o two-level namespaces. When rebinding a symbol, the logic should check if a specific library has been recorded for that symbol, and if the symbol is bound to a library, then never rebind symbols that happen to be the same name but from other libraries. See <mach-o/nlist.h>.