`hiredis` bindings

[casperisfine]

While redis-client is noticeably faster than the redis gem with the default "ruby driver", the "hiredis driver" still has a noticeable performance edge on large responses and I doubt we can't do much about it.

Ultimately parsing line protocols in Ruby involve a lot of string allocations and copying, which really shows on benchmarks.

I think it would make sense for redis-client to have a native extension to bind against hiredis when installed on MRI platform.