impl Reader<Reader<File>>

Hello Andrew!

First of all, thank you for this library and all other awesome work that you do!
I was browsing through sources of this crate to get a better understanding of how everything works and found something odd in [reader.rs:789](https://github.com/BurntSushi/rust-csv/blob/41c71ed353a71526c52633d854466c1619dacae4/src/reader.rs#L789).

```rust
impl Reader> {
```

I've spent quite some time trying to understand the intent here, but luckily we have Cargo so I ended up cloning the project and experimenting with it. It turns out that the project will build and test pass if we remove inner `Reader` or replace it with any other type.

This silliness also works:
```rust
impl Reader>>> {
```
```rust
impl Reader> {
```

So I'm assuming that this is just a typo that curiously slipped through the type checker.

I think it should be fixed to be just `impl Reader { ...` to not scare Rust newbies like myself.