Add doctests for union, diff, sym_diff, and intersection.

Add a rustdoc test for union to exhibit how it is used.

There is already a test for union in the test namespace, but this commit
adds a doctest that will appear in the rustdocs.

Add a doctest for the difference function.

Add a doctest for the symmetric_difference function.

Add a doctest for the intersection function.

Update the union et al. doctests based on @Gankro's comments.

Make the union et al. doctests a bit more readable.
Utilize fewer reexports

In regards to:

https://github.com/rust-lang/rust/issues/19253#issuecomment-64836729

This commit:

* Changes the #deriving code so that it generates code that utilizes fewer
  reexports (in particur Option::* and Result::*), which is necessary to
  remove those reexports in the future
* Changes other areas of the codebase so that fewer reexports are utilized
auto merge of #19407 : frewsxcv/rust/rm-reexports, r=cmr

In regards to:

https://github.com/rust-lang/rust/issues/19253#issuecomment-64836729

This commit:

* Changes the #deriving code so that it generates code that utilizes fewer
  reexports (in particur Option::\*, Result::\*, and Ordering::\*), which is necessary to
  remove those reexports in the future
* Changes other areas of the codebase so that fewer reexports are utilized
auto merge of #19488 : jbranchaud/rust/add-btree-set-doctests, r=alexcrichton

There is already a test for `union` in the test namespace, but this commit adds a doctest that will appear in the rustdocs.

Someone on IRC said, *Write doctests!*, so here I am.

I am not sure this is the best way to demonstrate the behavior of the union function, so I am open to suggestions for improving this. If I am on the right track I'd be glad to include similar doctests for `intersection`, `difference`, etc.
Merge remote-tracking branch 'upstream/master'