chore: Fix examples not working with cargo run (#998)

* chore: Fix examples not working with `cargo run`

## Motivation

PR #991 moved the `tokio` crate to its own subdirectory, but did not
move the `examples` directory into `tokio/examples`. While attempting to
use the examples for testing another change, I noticed that #991 had
broken the ability to use `cargo run`, as the examples were no longer
considered part of a crate that cargo was aware of:

```
tokio on master [$] via 🦀v1.33.0 at ☸️ aks-eliza-dev
➜  cargo run --example chat
error: no example target named `chat`

Did you mean `echo`?
```

## Solution

This branch moves the examples into the `tokio` directory, so cargo is
now once again aware of them:

```
tokio on eliza/fix-examples [$] via 🦀v1.33.0 at ☸️ aks-eliza-dev
➜  cargo run --example chat
   Compiling tokio-executor v0.1.7 (/Users/eliza/Code/tokio/tokio-executor)
   Compiling tokio-reactor v0.1.9
   Compiling tokio-threadpool v0.1.13
   Compiling tokio-current-thread v0.1.6
   Compiling tokio-timer v0.2.10
   Compiling tokio-uds v0.2.5
   Compiling tokio-udp v0.1.3
   Compiling tokio-tcp v0.1.3
   Compiling tokio-fs v0.1.6
   Compiling tokio v0.1.18 (/Users/eliza/Code/tokio/tokio)
    Finished dev [unoptimized + debuginfo] target(s) in 7.04s
     Running `target/debug/examples/chat`
server running on localhost:6142
```

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

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

README.md

@@ -98,7 +98,7 @@ fn main() {
 }
 ```
 
-More examples can be found [here](examples).
+More examples can be found [here](tokio/examples).
 
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