Remove generics of random state from GeneratorFunction

This changesets successfully remove generics of random state annotation from GeneratorFunction type.
As the result, when users work with fsi, they need not type many generics with GeneratorFunction.

Before the changes:

```fsharp
// error: inferred as generic function
> let g = Statistics.normal (0.0, 1.0);;
// ok
> let g<'s> = Statistics.normal (0.0, 1.0);;
```

After:

```fsharp
// ok
> let g = Statistics.normal (0.0, 1.0);;
```