Summary
=======

Replace storing `TLib` objects as `ref`s in `TSym` with storing all
`TLib` instances belonging to a module in a sequence that a `TSym` then
stores an index into.

This slightly reduces the size of `TSym` and, more importantly, makes
serializing the symbols and associated `TLib` instances easier: instead
of storing a `PackedLib` for *each* packed symbol, the packed symbols
now only store an index into the attached-to module's packed lib list.

Details
=======

For simplicity, the list with the libs for a module is stored in a
separate `seq` in `ModuleGraph`. The list cannot be stored in
`PContext`, as it needs to be accessible after semantic analysis, at
which point `PContext` is already gone.

Storing the `TLib` objects for a module happens when the module is
closed. While not strictly necessary at the moment, this is a
preparation for storing a `PSym` in `TLib.name` that is only set
up once the module is closed.

Aside from reducing the size of rodfiles, not storing a `PackedLib`
for each `PackedSym` also fixes another inefficiency, namely that
each deserialized got a unique, possibly duplicated, `TLib`
instance.

A symbol can now reference the `TLib` belonging to a module that is not
the one the symbol is attached to. This works around an issue with
aliased structural types.

Also, use the ID provided by the module's ID generator instead of the
`skModule`s position. While both represent the same thing (a
``FileIndex``), querying the ID generator is more consistent with how it
works elsewhere.

A module ID isn't necessarily the same before and after loading/storing
a rodfile. Loading and storing of `LibId`s now considers this.