Add a helper for compressing a slice into a vec

A common use case is compressing a `&[u8]` into a `Vec` without doing any I/O. The current docs don't make it clear how to do this.

A google search turns up [this forum post](https://users.rust-lang.org/t/brotli-compression/51645) with this code snippet:

```
use std::io::Write;

pub fn compress(input: &[u8]) -> Vec {
let mut writer = brotli::CompressorWriter::new(
Vec::new(),
4096,
11,
22);
writer.write_all(input).unwrap();
writer.into_inner()
}
```

Which is nontrivial to figure out. A helper functions in the root of the crate to compress a `&[u8]` into a `Vec` and to decompress a `Vec` into a `&[u8]` would help a lot with this. (Also, if these functions don't do I/O then perhaps they could be be infallible.)