Make rand(Integer) returns Integer

[soutaro]

Closes #1982

The rand implementation is a bit annoying for type checkers.

rand(0)    # Returns a Float
rand(1)    # Returns an Integer

The current type definition is sound that rand(1) returns Integer | Float, while it's really annoying because it returns Integer actually.

To make the type checkers more useful and match with our intuition, this PR makes the rand(1) returns Integer.

We assume the following pattern:

• When we really want an Float result, we usually pass 0 literal to rand -- rand(0) should return an Float
• When we pass some unknown integer value, we usually confirm the value is not 0, because we don't want Float result -- rand(i) can return Integer, no Float

The type definition uses literal type 0 to make type of rand(0) call Float. And other Integer cases returns an Integer.

This is unsound, but we believe this is more practical and useful.