For all symbols read from object files as well as generated from Zig code
will now be interned and have their offset into the string table saved on the `Symbol` instead.

Besides interning, local symbols now also use a decl's fully qualified name.
When a decl/symbol is extern/to-be-imported, the name of the decl itself will be used for symbol resolving.
Similarly for symbols that will be exported, will have their 'export name' set.