math/rand: add Reader variable

[dsnet]

The crypto/rand has both a Read function and a Reader object for passing in as an io.Reader. The math/rand package only has a Read function and no Reader object.

Should we add a Reader to math/rand to make it consistent with crypto/rand?

[bradfitz]

I thought this was a dup, but I was thinking of #8330 which went most of the way, but not all the way.

I still think there was an open bug or thread about this, but I don't remember the outcome.

[dsnet]

When #8330 was first implemented, I noticed that it didn't include an implementation for a Reader object. Back then, I didn't speak up because it's not a big deal.

I think it'd be nice, but everytime I really wanted to use random Reader, I just do rand.New(rand.NewSource(0)) and move on in life.