Improve RedisPipeline Results Ergonomics

[Mordil]

Right now - the RedisPipeline.execute() method resolves [RESPValue] - but this creates an unwieldy API compared to how RedisClient in general behaves.

In enqueue(operation:) the eventual resolves value is just ignored and discarded - but it would be nice to be able to capture the type information to somehow instead return [RESPValueEncodable] or have a way to grab in a reliable, non-optional, type-safe way the actual return values from the RedisClient interface so that it's consistent.

Right now, commands like zpopmax(from:max:) have a relatively complex response structure that users of RedisPipeline are left to figure out themselves, lest we just expose our helper method _mapSortedSetResponse(_:scoreIsFirst:)

[Mordil]

As part of PR #36 RedisPipeline has been removed from NIORedis.

At a later point, a batching API could be explored again - but it is being left up to higher level libraries at this time.