Improve debugging of `RESPValue`

Motivation:

Reading output from `RESPValue` existentials is entirely verbose and does not provide a human-readable description of what is being represented.

Modifications:

- Add conformance to `CustomStringConvertible` for `RESPValue`
- Remove redundant logging of arguments in `RedisConnection.send`

Result:

String representations of `RESPValue` should now be more readable for human to understand.