FieldIterator

    pub const FieldIterator = struct {
        next_idx: u32,
        names: []const NullTerminatedString,
        value_body_lens: ?[]const u32,
        bodies_overlong: []const Inst.Index,
        pub const Field = struct {
            idx: u32,
            name: NullTerminatedString,
            value_body: ?[]const Inst.Index,
        };
        pub fn next(it: *FieldIterator) ?Field {
            const idx = it.next_idx;
            if (idx == it.names.len) return null;
            it.next_idx += 1;
            return .{
                .idx = idx,
                .name = it.names[idx],
                .value_body = it.body(if (it.value_body_lens) |l| l[idx] else 0),
            };
        }
        fn body(it: *FieldIterator, len: u32) ?[]const Inst.Index {
            if (len == 0) return null;
            const b = it.bodies_overlong[0..len];
            it.bodies_overlong = it.bodies_overlong[len..];
            return b;
        }
    }