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.
This commits permits passing in static archives using the system
lib flag `-la`. With this commit, `zig ld` will now look firstly for
a dynamic library (which always takes precedence), and will fall back
on `liba.a` if the dylib is not found. The static archive is searched
for in the system lib search dirs like the dylibs.
Store only globals and undefs at the linker level, while all locals
stay scoped to the actual object file they were defined in. This is
fine since the relocations referencing locals will always be resolved
first using the local symbol table before checking for the reference
within the linker's global symbol table.
This also paves the way for proper symbol resolution from within static
and dynamic libraries.
