zld: introduce Stub.zig which represents parsed stub file

Instead of trying to fit a stub file into the frame of a Dylib struct,
I think it makes more sense to keep them as separate entities with
possibly shared interface (which would be added in the future).

This cleaned up a lot of logic in Dylib as well as Stub. Also, while
here I've made creating actual *Symbols lazy in the sense Dylib and
Stub only store hash maps of symbol names that they expose but we
defer create and referencing given dylib/stub until link time when
a symbol is actually referenced. This should reduce memory usage
and speed things up a bit.

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