Fixes the first case of #19340.

Now that we have an overloaded comparison (`==`) operator, and that `Vec`/`String` deref to `[T]`/`str` on method calls, many `as_slice()`/`as_mut_slice()`/`to_string()` calls have become redundant. This patch removes them. These were the most common patterns:

- `assert_eq(test_output.as_slice(), "ground truth")` -> `assert_eq(test_output, "ground truth")`
- `assert_eq(test_output, "ground truth".to_string())` -> `assert_eq(test_output, "ground truth")`
- `vec.as_mut_slice().sort()` -> `vec.sort()`
- `vec.as_slice().slice(from, to)` -> `vec.slice(from_to)`

---

Note that e.g. `a_string.push_str(b_string.as_slice())` has been left untouched in this PR, since we first need to settle down whether we want to favor the `&*b_string` or the `b_string[]` notation.

This is rebased on top of #19167

cc @alexcrichton @aturon 

Implement the `BitOr`, `BitAnd`, `BitXor`, and `Sub` traits from `std::ops` for TreeSet.  The behavior of these operator overloads is consistent with [RFC 235](https://github.com/rust-lang/rfcs/blob/master/text/0235-collections-conventions.md#combinations).

r? @Gankro

Fixes #19367.

Closes #18959

Technically, this causes code that once compiled to no longer compile, but
that code probably never ran.

[breaking-change]

------------

Not quite sure the error message is good enough, I feel like it ought to tell you "because it inherits from non-object-safe trait Foo", so I've opened up a follow-up issue #19538

Fixes #19498

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.

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