Clean up HashMap/HashSet examples

[killercup]

Sweeten the two main HashMap/HashSet examples from here and here with some deref and loop sugar.

(I've only tested this using this playpen.)

[rust-highfive]

r? @brson

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[killercup]

I've also included a small fix to shorten the HashSet description on http://doc.rust-lang.org/std/collections/#structs to only show the first sentence (the rest is a separate paragraph now).

[bluss]

Shouldn't this say PartialEq, Eq, Hash? If you want to you can change that too.

[killercup]

Added PartialEq (for HashSet as well).

[alexcrichton]

@bors: r+ 2ac380a rollup

Thanks!