Simplifies the airStructFieldPtr(index) functions to only obtain the
correct struct type and field index, which is then passed into the
structFieldPtr function. This function now calculates the byte-offset
of the field's address and returns a new `WValue` with this offset.
This means we only have to do this calculation in a single function,
and no longer have to duplicate any logic. This also handles both
regular (tagged) unions and packed unions.