Add initial implementation of "many sender" concept

[lewissbaker]

Add a "many sender" concept that can send a sequence of calls to a "many receiver":

• zero or more set_next() calls followed by;
• a call to one of set_value(), set_done() or set_error().

Add a bulk_schedule(scheduler, count) algorithm that sends count calls to set_next(r, idx). This should have a default implementation in terms of schedule(scheduler)

Add a get_execution_policy(receiver) CPO that allows a many-sender to query whether the receiver is able to support unsequenced/concurrent invocations of set_next().

Add some basic many-sender algorithms.

• reduce()
• for_each()
• transform()
• to_vector()