fix: Replace these with `BorshSerialize(Async)::serialize`

[DanikVitek]

issue: plz create a todo issue to replace these with BorshSerialize(Async)::serialize (not in the scope of this pr)

Originally posted by @dj8yfo in #337 (comment)

[dj8yfo]

this depends on yet unmerged code in #337 but is otherwise a completely valid task