Clean up the memory model

[rpj]

Even though RedisCommand is currently an internal API, the immediate-dereference-on-return pattern is terrible and needs be removed. I'm ashamed I wrote it this way in the first place, my bad.

[rpj]

Also, as noted in #53, the String class should be replaced with something less-fragmentary. Looping that task into this issue as they're contextually related and arguably excising use of String should be higher priority for memory-model cleanup anyway!