Remove costly assert from RandIntGen

Restrict the type parameters instead.

Before:

```
julia> @time for i = 1:10000000; rand(1:4); end
elapsed time: 1.144407975 seconds (160000000 bytes allocated, 5.39% gc time)
```

After:

```
julia> @time for i = 1:10000000; rand(1:4); end
elapsed time: 0.259933851 seconds (0 bytes allocated)
```

cc @ivarne @rfourquet