Add more types for Random

[zofrex]

This is my first time writing RBS so I am not sure it is right, especially the tests.

I am not sure how to fully describe rand because it can return whatever type you give it, for example:

> Random.rand(Tally.new("|")..Tally.new("|||||"))
=> #<Tally:0x00007fedf725f9f8 @string="|">

In other words, rand: (Range[SomeType]) -> SomeType but I don't know how to write that in RBS.

(Also, I think SomeType might have to be a descendent of Numeric, but the documentation is not clear)