Back io::stdin with a global singleton BufferedReader

io::stdin returns a new `BufferedReader` each time it's called, which
results in some very confusing behavior with disappearing output. It now
returns a `StdinReader`, which wraps a global singleton
`Arc<Mutex<BufferedReader<StdReader>>`. `Reader` is implemented directly
on `StdinReader`. However, `Buffer` is not, as the `fill_buf` method is
fundamentaly un-thread safe. A `lock` method is defined on `StdinReader`
which returns a smart pointer wrapping the underlying `BufferedReader`
while guaranteeing mutual exclusion.

Code that treats the return value of io::stdin as implementing `Buffer`
will break. Add a call to `lock`:

```rust
io::stdin().read_line();
// =>
io::stdin().lock().read_line();
```

Closes #14434

[breaking-change]