Since `packed` containers are now internally represented by a `bitpack`,
they need special handling on initialization: they need to be either
bitpacked or bitcasted to their backing integer. `Sema` already did this,
but `LowerZon` didn't yet.
This commit allows using ZON (Zig Object Notation) in a few ways.
* `@import` can be used to load ZON at comptime and convert it to a
normal Zig value. In this case, `@import` must have a result type.
* `std.zon.parse` can be used to parse ZON at runtime, akin to the
parsing logic in `std.json`.
* `std.zon.stringify` can be used to convert arbitrary data structures
to ZON at runtime, again akin to `std.json`.
Justus Klausecker