Change type::fields to return an array_view

Change type::fields to return an array_view

This patch changes type::fields to return an array_view of the fields,
then fixes up the fallout.

More cleanups would be possible here (in particular in the field
initialization code) but I haven't done so.

The main motivation for this patch was to make it simpler to iterate
over the fields of a type.

Regression tested on x86-64 Fedora 41.

Approved-By: Simon Marchi <simon.marchi@efficios.com>