Make return value of Serializer.lazily lazy (#2627) (#2628)

Directly subclassing Iterable is lazy-ish, but if you call any operation
on the resulting value (eg. map or ++) it will evaluate the Iterable and
return a List.

(cherry picked from commit 57b8a395

)

Co-authored-by: Jack Koenig <koenig@sifive.com>