Explain closures much better in the tutorial

[mdinger]

Basically, I didn't think the tutorial helped much at all to explain closures so I tried to understand it and make changes so that it is clearer. Guys on IRC helped answer my questions so I think hope what I changed is pretty accurate.

I'll probably try to do more but I wanted to put this up so people could see. I think it's much better. They're all in this commit.

mdinger@b580964

Let me be clear, from reading the tutorial earlier, it wasn't obvious b: |int| had anything to do with a function. I thought it meant an int was gonna be assigned to b. All the vertical bars did was make it special.

[flaper87]

part of #11755

[mdinger]

I think this should be much better than before. Perhaps some more complex examples should be included.

I tried to get their stack closure example to println!() but it complained about ownership. Stack closure is also mentioned as one alternative of many but they don't specify any others. I asked on IRC and was basically told that closures alternatives are subject to change.

mdinger@d67b1b2