exec::nest_scopes

Similar to exec::enter_scopes except the scopes are not entered in
parallel. The first enter scope sender's operation runs, then the
second, et cetera. The yielded exit scope sender exits the scopes in the
reverse order. If at any point an enter scope sender's operation ends
with error or stopped all scopes that have been entered are exited in
reverse order and the operation completes with the error or stopped
signal received.

Author: RobertLeahy