Replace set_single_recipient with drain_to

Replace set_single_recipient with drain_to

What set_single_recipient does turns out to be useful with multiple
recipients.
Effectively, set_single_recipient was simply creating a change
output that was arbitrarily required to be the only output.
But what if you want to send excess funds to one address but still have
additional recipients who receive a fixed value?
Generalizing this to `drain_to` simplifies the logic and removes several
error cases while also allowing new use cases.

"maintain_single_recipient" is also replaced with "allow_shrinking"
which has more general semantics.