Update method signatures to use generic arg

Replace the `rdd` argument in all of the S4 methods with `x`. This will allow us to standardize the code as other Spark components get added and we need to set up multiple dispatch on S4 methods.

In any cases where `x` was used as a generic iterator, I've replaced it with `i` except in a few cases where a different letter made sense. For example, in some of the pair functions, we now use `function(k)` and `function(v)` for the key/value functions.