docs: expand and update crate-level docs (#1806)

## Motivation

Tokio's crate-level docs are currently pretty sparse, and in some cases
reference old names for APIs. Before 0.2 is released, they could use a
fresh coat of paint.

## Solution

This branch reworks and expands the `lib.rs` docs. In particular, I've
added a new "A Tour of Tokio" section, inspired by the [standard
library's similarly-named section][std]. This section lists all of
`tokio`'s public modules, and summarizes their major APIs. It also lists
the feature flags necessary to enable those APIs.

[std]: https://doc.rust-lang.org/std/index.html#a-tour-of-the-rust-standard-library

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

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