NSDictionary uses only NSStrings

[Pandawan]

This issue is a carry-over from a discussion in #16.

I'm unsure if this is a limitation of the objective c to rust library or if I missed something, but I was unable to create an NSDictionary with multiple value types. As a result, I was forced to stringify arrays, numbers, and booleans to pass them to the objective c side.

This works but ends up in ugly/hackish code where all the data is stringified before being passed between rust and objective c.

I was thinking that we could potentially use C unions to pass data of any kind, but I'm unsure of the consequences of doing this.